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Ind. Eng. Chem. Res. 2009, 48, 2066–2075
Activated Carbons for the Removal of Low-Concentration Gaseous Toluene at the Semipilot Scale Juan Carratala´-Abril, Maria Angeles Lillo-Ro´denas, Angel Linares-Solano, and Diego Cazorla-Amoro´s* Grupo de Materiales Carbonosos y Medio Ambiente, Departamento Quı´mica Inorga´nica, Facultad de Ciencias, UniVersidad de Alicante, Apartado 99, E-03080, Alicante, Spain
The present paper focuses on the removal of low-concentration gaseous toluene by adsorption on activated carbons (AC) at the semipilot plant scale. The performance of AC prepared by chemical and physical activation is analyzed and compared with a commercial AC. Our results have shown similar shapes for the breakthrough curves in the physically AC and the commercial AC, close to ideality, and similar lengths of the mass transfer zone, despite their different pore size distributions. This behavior is mostly related to a strong adsorbate-adsorbent interaction for the three AC, whereas this interaction has proved to be weaker in the chemically activated ones. The narrow micropore volume influences the breakthrough and saturation times, and a simple kinetic model has been used to fit the experimental data leading to good prediction of the adsorption capacities. A mathematical model, suitable for diluted concentrations, has been used to simulate the breakthrough curves. This model has proved to be useful to fit curves at laboratory scale and predict the adsorption behavior at larger scales. 1. Introduction Adsorption on activated carbons (AC) is one of the most widely used methods for the removal of pollutants from water and gaseous streams.1 Among the different pollutants, volatile organic compounds (VOC) should be underlined due to their toxicity.2,3 The effectiveness of AC on the retention of such pollutants has led to a growing demand over the years, and several studies have focused on low-concentration VOC adsorption.3-8 Despite that, only recent studies have analyzed the role of porosity and surface chemistry of the activated carbons on low-concentration gaseous VOC adsorption.9-15 For these studies, at laboratory scale, a wide range of activated carbons obtained from different preparation methods and differing in their porosities, pore size distributions, and surface chemistry was selected. These studies proved that AC with high narrow micropore volumes and a low content in surface oxygen groups are desired to achieve high benzene and toluene adsorption capacities at 200 ppmv.9-11 From previous results, the performance of chemically activated carbons should be underlined in terms of adsorption capacities and breakthrough times.10 However, and despite the interest this topic has attracted, there are very few studies on low-concentration VOC adsorption at larger scales, that is at semipilot or even pilot plant scale. Hence, the aim of the present study, taking into account the good behavior at the laboratory scale of chemically activated carbons,9-11 is to extend our previous results to a larger scale, analyzing the performance of materials with different origins and porosities. These AC are obtained in powder form as a result of the activation process, and for a large scale application, they are required in granular form. Hence, this work is also devoted to their agglomeration and to study if such an agglomeration process modifies their performance. Comparison between adsorptions on the AC has been performed, in order to analyze the role of porosity and pore size distribution on toluene adsorption. A mathematical model previously described for * To whom correspondence should be addressed. Tel.: +34965903946. Fax: +34965903454. E-mail:
[email protected].
diluted concentrations16,17 has been applied to the adsorption data at both scales in order to check its suitability for low-concentration VOC adsorption simulation. Finally, comparison between parameters from laboratory and semipilot systems has been done to determine if the performance of large scale systems can be predicted from the parameters obtained in laboratory experiments. Thus, the results from the present paper are of great interest for scaling up low-concentration VOC adsorption processes. 2. Experimental Section 2.1. Activated Carbons. Adsorption studies were performed using activated carbons with different porous textures (volumes and pore size distributions). They comprise a commercial activated carbon from MeadWestvaco, activated carbons prepared by chemical activation using alkaline hydroxides, and two activated carbons prepared by steam activation from a subbituminous coal at different burnoff percentages. The reference for the commercial activated carbon from MeadWestvaco is WV-A1100 10 × 25 mm, and its mean particle size is 1.3 mm. The mean particle size is used as the particle diameter (dp) in the calculations. Chemically activated carbons were prepared using a physical mixing procedure detailed elsewhere,18 but were prepared on a larger scale. This procedure can be summarized as follows: a physical mixture of the alkaline hydroxide (NaOH or KOH) and the carbon precursor is prepared and pyrolyzed in nitrogen flow at 5 K/min up to 1000 K, and then held for 1 h. Nitrogen flow rates from 1500 to 6000 mL/min were used. The weight of carbon precursor used each time was 6 or 60 g, and the hydroxide/precursor weight ratio studied was 3/1. The resulting activated carbons were in powder form. Some of the prepared AC were agglomerated. The agglomeration procedure is described by the following: The activated carbon was mixed with 11% humic acids solution (1 g of AC per 10 mL of solution) until preparation of a homogeneous suspension, which was dried in a furnace at 383 K for 12 h and ground to get a fine powder. It was pressed for 5 min at 1300 kg/cm2, leading to carbon
10.1021/ie800521s CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2067
Figure 1. (a) Chemically activated carbon in powder form, (b) activated carbon in carbonized monoliths, and (c) chemically activated carbon in granular form (1.3 mm).
Figure 2. Nitrogen adsorption isotherms at 77 K of the activated carbons studied. Table 1. Activated Carbons Used in the Adsorption Experiments nomenclature WV-A1100 K3-1500 Na3-6000p K3-6000p physically AC (A) physically AC (B)
precursor wood anthracite anthracite anthracite subbituminous subbituminous
preparation method chemical activation (H3PO4) chemical activation (KOH) chemical activation (NaOH) chemical activation (KOH) physical activation (steam) physical activation (steam)
Table 2. Porous Texture Characterization of the Activated Carbons
shape granular powder granular granular granular granular
monoliths. They were dried at 383 K for 12 h and carbonized in nitrogen up to 923 K, and then held for 1 h at 75 mL/min · g nitrogen flow rate. The monoliths were then ground and sieved to get granular activated carbons with a uniform size of 1.3 mm, approximately. Nomenclature used for the chemically activated carbons can be summarized as Xn-N, where “X” corresponds to the activating agent (Na, sodium; K, potassium), “n” corresponds to the hydroxide/carbon ratio, and “N” is a number which indicates the nitrogen flow rate used during pyrolysis in milliliters per minute. Thus, Na3-1500 is an activated carbon prepared by sodium hydroxide using a 3/1 NaOH/precursor ratio and pyrolyzed in 1500 mL/min N2 flow rate. The nomenclature of the chemically activated AC which have been agglomerated, ground, and sieved up to a size of 1.3 mm includes “p”. Figure 1 presents a chemically activated carbon as prepared (in powder form) (a), the activated carbon in carbonized monoliths (b), and the final activated carbon in granular form (1.3 mm) (c). The physically activated carbons were prepared by steam activation using a subbituminous coal precursor from TECSA (Puertollano, Ciudad Real). The steam temperature was, in both cases, 1073 K, and different holding times were used for samples physically AC A and physically AC B, 1.7 and 4 h, respectively. With such activation conditions the burnoff percentages were 36 and 63%, respectively. The mean particle size of these granular activated carbons was also in the range of 1.3 mm.
sample WV-A1100 K3-6000p K3-1500 Na3-6000p physically AC (A) physically AC (B)
SBET VDR-N2 VDR-CO2 (m2/g) (cm3/g) (cm3/g) 1757 2360 1987 1891 883 475
0.67 0.89 0.86 0.70 0.35 0.22
0.36 0.57 0.66 0.46 0.26 0.21
microporous VDR-N2 contribution VDR-CO2 (%) (cm3/g) 54 79 86 80 74 86
0.31 0.32 0.20 0.24 0.09 0.02
Table 3. Density and Porosity Characterization of the Activated Carbons sample
Fb (kg/m3)
Fp (kg/m3)
Fs (kg/m3)
εb
εp
WV-A1100 K3-6000p K3-1500 Na3-6000p physically AC (A) physically AC (B)
270 220 290 320 570
590 550 570 630 850
1990 2000 1830 1970 1500
0.54 0.60 0.49 0.49 0.33
0.71 0.72 0.69 0.68 0.43
Table 1 compiles the AC used together with their nomenclature, precursors, preparation methods, and shapes. 2.2. Porosity Characterization of the Activated Carbons. Porosities of the AC were characterized by physical adsorption of gases (N2 at 77 K and CO2 at 273 K) using an Autosorb-6B apparatus from Quantachrome. The BET equation was applied to the nitrogen adsorption data to get the apparent BET surface area (SBET). The DubininsRadushkevich equation was applied to the nitrogen adsorption data to determine the total micropore volume (pores with size
