Highlighting the Role of Activated Carbon Particle Size on CO2

Article pubs.acs.org/IECR

Highlighting the Role of Activated Carbon Particle Size on CO2 Capture from Model Flue Gas Marco Balsamo,† Francisco Rodríguez-Reinoso,‡ Fabio Montagnaro,*,§ Amedeo Lancia,† and Alessandro Erto† †

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ‡ Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica-Instituto Universitario de Materiales, Ap. 99, E-03080 Alicante, Spain § Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, 80126 Napoli, Italy ABSTRACT: CO2 adsorption onto two particle size classes of the commercial activated carbon Filtrasorb 400, namely 600−900 μm (sample F600−900) and 900−1200 μm (sample F900−1200), was investigated at 293 K under model flue gas conditions in a fixed-bed column. Equilibrium adsorption capacity for a typical 15% CO2 postcombustion effluent was 0.7 mol kg−1 for both investigated adsorbents. In both cases, CO2 breakthrough curves showed a reduction of the characteristic breakpoint time and faster capture kinetics at higher pollutant concentration in the feed (in the range 1−15%). Dynamic adsorption data highlighted the important role played by wider micropores in determining a quicker adsorption process for finer particles. Mathematical modeling of the 15% CO2 breakthrough curve allowed identifying intraparticle diffusion as the limiting step of the adsorption process. Numerical analysis provided values of the intraparticle mass-transfer resistances equal to 1.7 and 3.3 s for F600−900 and F900−1200, respectively.

1. OVERVIEW The scientific community agrees that anthropogenic CO2 emission, mainly deriving from fossil-fueled power plants, is among the main contributors to global warming.1,2 Different options are available to mitigate CO2 emissions deriving from the power sector, including use of noncarbon fuels (hydrogen and renewable energy), higher power generation efficiency, development of innovative energy production systems, such as oxycombustion and chemical-looping combustion, and the adoption of efficient technologies for CO2 capture and storage (CCS).1,3 Postcombustion CO2 capture processes (absorption, adsorption, membrane purification, and cryogenic distillation) have the greatest near-term potential for reducing greenhouse gas emissions because these processes can be retrofitted to existing units, thus providing a quicker solution to mitigate CO2 environmental impacts.2,4 The main barrier to the implementation of these technologies on an industrial scale is related to the low thermodynamic driving force for CO2 capture from flue gas.2 Postcombustion chemical absorption of CO2 in aqueous amine solutions, mainly monoethanolamine (MEA), is the most widely used separation technology.5,6 The MEA process suffers many drawbacks related to the considerable amounts of thermal energy required for absorbent regeneration, the high equipment corrosion rate caused by contact with MEA solution, and the solvent degradation caused by oxygen and oxygen-based compounds such as SO2 and NOx present in a typical flue gas.5,7 As a consequence of the aforementioned issues, several research groups are making great efforts to develop highperformance and cost-effective CO2 advanced separation processes to accelerate the techno-economic feasibility of postcombustion capture systems. In this scenario, adsorption © 2013 American Chemical Society

represents an attractive purification technology widely used for the treatment of gaseous and liquid effluents due to its high operating flexibility, general low maintenance costs, and if coupled with an effective regeneration process, the absence of byproducts.8−17 For CCS applications, many sorbents can be used either raw or functionalized.9,13,18,19 Activated carbons show high potentiality for application in CO2 capture because they are generally less costly than other adsorbents (e.g., zeolites, ordered mesoporous silicas, metal organic frameworks, etc.) and have a complex structure characterized by high surface area and tunable porosity/surface properties.20−23 In addition, carbonbased sorbents are easily regenerable allowing their use in processes such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), and vacuum swing adsorption (VSA).24,25 Despite these advantages, CO2 removal performances of activated carbons under typical flue gas conditions (CO2 1−15% by vol and atmospheric pressure) have been poorly investigated.26,27 Moreover, the role played by different microstructural properties of carbon-based materials (particle size and pore size distribution) on the solids CO2 capture capacity and dynamic adsorption behavior has been only partially elucidated.22,26 Wahby et al.22 highlighted that carbon molecular sieves characterized by a high volume of narrow micropores and with a narrow pore size distribution exhibited high CO2 adsorption capacity. Plaza et al.26 reported that essentially microporous activated carbons displayed higher mass-transfer Received: Revised: Accepted: Published: 12183

June 7, 2013 July 31, 2013 July 31, 2013 July 31, 2013 dx.doi.org/10.1021/ie4018034 | Ind. Eng. Chem. Res. 2013, 52, 12183−12191

Industrial & Engineering Chemistry Research

Article

Figure 1. Layout of the experimental apparatus.

commercialized as N2Gsorb-6 (www.g2mtech.com), working at 77 and 273 K for N2 and CO2, respectively. Adsorption measurements were recorded in the relative pressure (P/P0) range 10−7 to 1 for nitrogen and a range 10−7 to 0.03 for carbon dioxide. Prior to adsorption runs, each sample was degassed under vacuum at 373 K to remove humidity or other volatile impurities. The apparent surface area (SBET) was derived by applying the Brunauer−Emmett−Teller (BET) equation to N2 adsorption isotherm in the P/P0 range 0.01 to 0.15. The total micropore volume (V0) was obtained from N2 adsorption data using the Dubinin−Radushkevich (DR) equation, whereas the mesopore volume (Vmeso) was deduced as the difference between the total pore volume (Vt), corresponding to the amount adsorbed at P/P0 = 0.97, and V0. Finally, the volume of narrow micropores (Vn, pore width up to 0.7 nm) was evaluated from CO2 adsorption isotherm at 273 K using the DR equation.22 2.2. Lab-Scale Plant for CO2 Adsorption Experiments. Figure 1 shows a schematic representation of the experimental apparatus adopted for the execution of dynamic adsorption runs. The feed gas composition (N2 + CO2) was determined via mass flow controllers (EL-FLOW series 201-CV, Bronkhorst). CO2 adsorption tests on the investigated sorbents were carried out in a fixed-bed column (total length 0.13 m; inner diameter 0.02 m) made up of Pyrex glass, equipped with a 45 μm porous septum and composed of two units for adsorbent charging/discharging operations. The fixed-bed temperature was controlled by means of an ad hoc heating system, arranged coaxially with the adsorber unit. It consists of three 500 W cylindrical shell Watlow band heaters, enveloped in a thermal insulating layer of ceramic fibres, and connected to EZ-PM proportional integral derivative controllers (Watlow). Before dynamic tests, a calibration curve was built to establish a relationship among band heater surfaceband heater/fixed-bed interspace-adsorbent granular bed temperatures. Once the thermal profile was known, the fixed-bed temperature was defined during adsorption tests by setting and controlling the band heater/fixed-bed interspace temperature by means of type-J thermocouples. CO2 percentage volumetric concentration measurements during adsorption tests were carried out by a continuous nondispersive infrared AO2020 Uras 26 gas analyzer (ABB). Data acquisition and elaboration were performed by interfacing the analyzer with a PC unit via

resistance, for CO2 adsorption under postcombustion conditions, with respect to an activated carbon having an important contribution of meso- and macropores. This work aims at elucidating the effect of the porosimetric structures of two particle size classes of a commercial activated carbon on their CO2 capture capacity under typical flue gas conditions. The innovative aspect of the research also relies on providing new insights on relationships between sorbent microstructural properties and CO2 equilibrium and dynamic capture performances, the latter aspect being partially investigated in the literature but of crucial importance to assess the potential applicability of a sorbent for treating real flue gas streams. CO2 adsorption experiments from model flue gas streams were performed in a fixed-bed adsorber integrated in a lab-scale plant. Kinetic and thermodynamic CO2 adsorption results were interpreted in light of the sorbent chemical and textural properties derived from a thorough characterization analysis.

2. EXPERIMENTAL SECTION 2.1. Adsorbents and Characterization Techniques. The parent adsorbent selected in this work is the commercial activated carbon Filtrasorb 400 (F400, manufactured by Calgon Carbon) obtained from steam activation of a bituminous coal. Laser granulometric analysis of the parent material was performed by a Mastersizer 2000 granulometer (Malvern Instruments) operated down to a minimum particle size of 0.02 μm. Two particle size classes of F400 were selected for CO2 adsorption tests by mechanical sieving of the parent material, namely 600−900 μm (sample F600−900) and 900−1200 μm (sample F900−1200). It should be noted that particles finer than 600 μm were not investigated to avoid significant pressure drops across the fixed-bed column, whereas an upper limit of 1200 μm was fixed to ensure a column-to-particle radius ratio above 15, which allows us to neglect radial fluid velocity gradients in the fixed bed during adsorption experiments.28 Elemental analyses on F600−900 and F900−1200 adsorbents were carried out by means of a CHN-2000 analyzer (LECO Corporation). Porosimetric analyses for the selected particle size classes were performed in a homemade fully automated instrument designed and constructed by the Advanced Materials group (LMA), now 12184

dx.doi.org/10.1021/ie4018034 | Ind. Eng. Chem. Res. 2013, 52, 12183−12191


Article

LabView software. Gas volumetric flow rate variations, occurring in the fixed bed, were monitored by means of a mass flow controller EL-FLOW series 201-CV (working as a flow meter), set at the exit of the adsorption column, and digitally interfaced with the PC unit for an online data acquisition. A bypass line was also implemented in the lab-scale apparatus to verify the feed composition prior to adsorption experiments: a three-way ball valve (V1) allows us to adequately switch the gas flow while two ball valves (V2 and V3) are devoted to avoid back-flow toward the excluded line. 2.3. Fixed-Bed Dynamic Tests. Continuous adsorption experiments were performed by feeding the column with a 2.5 × 10−5 m3 s−1 gas stream, at 293 K and 1 bar total pressure, containing CO2 at percentage volumetric concentrations varied in the range 1−30% (balance N2). The column was charged with a known amount of F600−900 or F900−1200 activated carbons (m = 0.015 kg, previously heated overnight at 378 K to remove humidity). It is underlined that CO2 capture tests at pollutant concentrations greater than typical 15% flue gas (namely 25% and 30%) were performed to better interpret the qualitative trend of the adsorption isotherms. The dynamic behavior of the gas−solid adsorption system was followed by monitoring the pollutant percentage volumetric concentration at the fixed-bed outlet as a function of time, obtaining the breakthrough curves. More specifically, the time evolution of the composition profile was expressed in terms of the ratio of the volumetric CO2 flow rate at the bed outlet relative in to that in the feed, Qout CO2/QCO2. N2 flow rate was verified to be practically constant during all adsorption runs. Thus, N2 adsorption on both the F600−900 and F900−1200 adsorbents was considered negligible under the investigated experimental conditions. CO2 kinetic adsorption results at 293 K were processed to obtain the corresponding adsorption isotherms. The material balance on CO2 species over the adsorption column leads to the following expression for the equilibrium CO2 adsorbed amount ωeq: ωeq =

in ρ Q CO 2 CO2

MCO2m

∫0

teq

⎛ ⎜1 − ⎜ ⎝

out Q CO (t) ⎞ 2 ⎟ in ⎟ Q CO 2 ⎠

hypotheses were adopted for numerical analysis:29,31−34 (i) the flow pattern is described by the axial dispersion flow model, (ii) radial concentration and temperature gradients are negligible, (iii) the system is isothermal. It should be noted that typically employed physisorbents (such as activated carbons) show low heat adsorption, which therefore has a negligible effect on gas temperature variations. This should support the hypothesis of an isothermal process.9 The mass balance for the adsorbate in a differential element of the column is given by29 −εDax

ωeq = KFPeq

1/ n

ω(̅ t = 0, z) = 0

(5)

C(t = 0, z − {0}) = 0

(6)

C(t , z = 0) = C in

(7)

∂C ∂z

(8)

=0 (t , z = L)

The pollutant−material balance in a differential element of an adsorbent particle considered as spherical can be expressed as29 ρp

⎡ 2 ∂ω ∂ω ∂ 2ω ⎤ − ρp Dp⎢ + ⎥=0 ∂t ⎣ r ∂r ∂r 2 ⎦

(9)

in which Dp was assumed constant throughout the particle and evaluated as a fitting parameter for the dynamic mathematical model (vide infra). The resolution of the mass balance equation (9) can be obtained by fixing the following initial and boundary conditions: ω(t = 0, r) = 0

dt

ρp Dp

(1)

∂ω ∂r

∂ω ∂r

(10)

= (1 − ε)kext(C − C*) (11)

(t , r = R S)

=0 (12)

(t , r = 0)

It is assumed that thermodynamic equilibrium establishes at the gas phase−particle interface: ω(t , r = R S) = f (C*)

(13)

with f(C*) representing the adsorption isotherm (eqs 2 and 3). An estimation of the external mass-transfer coefficient kext can be obtained according to the Wakao and Funazkri correlation:34,35

ωmax KLPeq 1 + KLPeq

(4)

with the following initial and boundary conditions:

The value of the integral in eq 1, which corresponds to the area above the breakthrough curve for each investigated inlet CO2 concentration, was obtained by applying the trapezoidal rule to the experimental kinetic data. 2.4. Adsorption Thermodynamics and Kinetics Modeling. Adsorption isotherms have been interpreted in light of the well-known Langmuir (eq 2) and Freundlich (eq 3) models for a comprehension of the main mechanisms involved in the capture of CO2 by the investigated adsorbents:29,30 ωeq =

∂ 2C ∂uC ∂C ∂ω + +ε + (1 − ε)ρp ̅ = 0 ∂z ∂t ∂t ∂z 2

(2)

Sh = 2 + 1.1Re 0.6Sc1/3 (3)

(14)

where Re = dSρgu/μ, Sh = dSkext/DCO2−N2 and Sc = μ/ρgDCO2−N2 represent the dimensionless Reynolds, Sherwood, and Schmidt numbers, respectively. The value of DCO2−N2 at the operating temperature (293 K) was computed according to Chapman− Enskog equation.35 The axial dispersion coefficient in eq 4 can be evaluated as35

Freundlich and Langmuir parameters for F600−900 and F900− 1200 adsorbents were obtained from a nonlinear fitting of CO2 adsorption isotherms at 293 K. Mathematical modeling of the breakthrough curves was performed for a CO2 concentration in the feed equal to 15% vol which is representative of a typical flue gas composition. As a matter of fact, the estimation of mass-transfer parameters under this condition is more interesting for sizing an adsorption unit aimed at purifying a real flue gas stream. The following

Dax = 0.73DCO2 − N2 + 12185

0.5dSu

(

ε 1+

9.49ε ReSc

)

(15)



Article

which allows an estimation of the axial dispersion effect in the packed bed through the Péclet number Pe = uL/εDax. The aim of the adsorption kinetics modeling was to identify the rate-limiting step of the adsorption process, which was achieved by comparing the film and intraparticle diffusion resistances defined, respectively, as29,31 dS 6kext

(16)

d2 Ωp = S 60Dp

(17)

Ωext =

Table 1. Relevant Chemical and Microstructural Parameters Obtained for F600-900 and F900-1200 Adsorbents

granulometric analysis ultimate analysis

N2/CO2 porosimetric analyses

The numerical resolution of mass balance equations (4) and (9) was obtained with Aspen Adsim modeling environment adopting the method of lines. A Taylor-based upwind differencing scheme was used for the discretization of first-order spatial derivatives, and a second-order central differencing scheme was used for the discretization of the second-order terms. Finally, Aspen Adsim software enables the evaluation of Dp as a fitting parameter by minimizing the sum of the squared differences between numerically calculated and experimentally observed values of the gaseous phase composition at the fixed-bed outlet (least-squares method).

dS, μm C, wt% N, wt% H, wt% Vt, cm3 g−1 V0, cm3 g−1 Vn, cm3 g−1 Vmeso, cm3 g−1 SBET, m2 g−1

F600− 900

F900− 1200

766 85.94 0.06 0.04 0.58 0.41 0.32 0.17 1076

1050 86.15 0.11 0.02 0.52 0.33 0.32 0.19 902

N2 adsorption−desorption isotherms at 77 K obtained for sorbents F600−900 and F900−1200 are depicted in Figure 3.

3. RESULTS AND DISCUSSION 3.1. Adsorbents Microstructural and Chemical Properties. The absolute particle size distribution obtained for the parent F400 activated carbon is shown in Figure 2. As a general

Figure 3. N2 adsorption−desorption isotherms at 77 K for F600−900 and F900−1200 adsorbents.

The isotherms for both samples are very similar and they are type I according to IUPAC classification;37 the high N2 volume adsorbed observed at very low relative pressures (i.e., P/P0 < 10−3) testifies to prevailing microporous nature for the classes of F400.38 A comparison between the adsorption and desorption branches highlights the non-negligible presence of mesopores. The narrow hysteresis loop observed from isotherms is of type H4 according to IUPAC, commonly associated with the presence of slit-shaped pores.38 In addition, it can be observed that the volume of N2 adsorbed is higher for F600−900 mostly in the “knee” region of the adsorption isotherms (P/P0 < 0.1). This is a clue of a more developed microporous structure for the finer particle size class. The main textural parameters derived for F600− 900 and F900−1200 by working-out N2 and CO2 adsorption data at 77 and 273 K, respectively (cf. section 2.1) are listed in Table 1. The results confirm that both particle size classes are mainly microporous, with V0 being 71 and 63% of the total pore volume for F600−900 and F900−1200, respectively. Furthermore, the higher values of Vt and SBET derived for F600−900 can be ascribed to a greater contribution of wide micropores for this material, whereas coarser particles are characterized by a slightly greater contribution of mesopores. Very interestingly, the higher difference between V0 and Vn values derived for F600−900 (Vn are identical for both samples) denotes the occurrence of a broader micropore size distribution for this adsorbent with respect to F900−1200.39 3.2. Thermodynamic Aspects of CO2 Adsorption onto F600−900 and F900−1200. Figure 4 depicts the equilibrium

Figure 2. Absolute particle size distribution for parent F400 sample.

consideration, it can be observed that the sample particles belong to the range 1.3 μm to 1.9 mm. Moreover, F400 shows a substantially unimodal distribution that peaked at 1063 μm with a characteristic tail extending from 1.3 to 212 μm, due to the presence of finer particles. Using raw granulometric data, it was possible to determine values of the mean Sauter particle diameter36 for each particle size class selected in this work by normalizing the absolute distribution for each particle size range: dS resulted equal to 766 and 1050 μm for F600−900 and F900− 1200 adsorbents, respectively (cf. Table 1). Results obtained from ultimate analysis of F600−900 and F900−1200 solids are reported in Table 1. It is possible to show that, as expected, the samples show a prevailing carbonaceous matrix with a mean C-content of nearly 86%, whereas the presence of nitrogen and hydrogen is practically negligible. As a consequence, results highlight identical chemical compositions for the two adsorbents investigated. 12186



Article

(close to unity). The similarity of the parameters determined from each model for both adsorbents reflects the alreadydescribed trend of ωeq values. Finally, it is interesting to highlight that in both cases, the values of the Freundlich heterogeneity parameter 1/n do not differ too much from unity, which clearly indicates that the sorbent surfaces are practically energetically homogeneous in the CO2 capture process.30 3.3. Kinetic Aspects of CO2 Adsorption onto F600−900 and F900−1200. CO2 adsorption dynamic patterns of F600− 900 and F900−1200 adsorbents were experimentally determined at 293 K, as a function of the pollutant concentration in the feed. In Figure 5a−h the corresponding results are reported. Generally, it can be highlighted that both systems are characterized by fast adsorption kinetics, reaching equilibrium conditions (for which CO2 concentrations at the bed inlet and outlet are practically equal) in about 10 min in all cases. More interestingly, it can be observed that for each particle size class the breakthrough curves show shorter breakpoint times tb (time in for which Qout CO2/QCO2 = 0.05) and higher slopes of the linear part of the sigmoid as the CO2 initial concentration increases. In fact, tb is 1.7 times longer at 1% CO2 inlet concentration with respect to a 15% CO2 gas stream for both activated carbons. This behavior could be ascribed to an increase in mass-transfer rate occurring at a higher process driving force.29 Moreover, kinetic adsorption data at a fixed initial CO2 concentration show that the values of the breakpoint time and of the slopes of the sigmoid result greater for F600−900, thus indicating faster mass-transfer phenomena for this sample (vide infra). For example, tb is 47 and 44 s for F600−900 and F900−1200, respectively, for a typical 15% CO2 flue gas stream (cf. Table 3). The described experimental evidence shows the important role played by the different microstructural properties of the tested particle size classes in determining different CO2 capture kinetics. As a matter of fact, the presence of a broader micropore size distribution with wider micropores for F600−900 (cf. Table 1 and section 3.1) should determine faster diffusion rates of CO2 species in the adsorbent pores, despite the slightly lower contribution of mesopores. These results should be analyzed also in light of saturation (equilibrium) adsorption capacity, which is practically the same for the two activated carbons at each initial CO2 concentration (see section 3.2). The higher mass-transfer rate (slope of the sigmoid) observed for F600−900 determines a more efficient use of this adsorbent, because the adsorption capacity at the breakpoint time is greater with respect to F900−1200 (longer breakpoint time). The main fluid dynamic and kinetic parameters determined from the modeling analysis of breakthrough curves obtained for F600−900 and F900−1200 at 293 K and for a 15% CO2 gas stream, which is more interesting for practical applications, are reported in Table 3. Relevant fixed-bed properties are also listed for completeness. Figure 6 compares the experimental and theoretical breakthrough curves for the investigated activated carbons determined at 15% CO2 concentration and 293 K. It is stressed that the Freundlich thermodynamic model was adopted as the equilibrium isotherm in eq 13 (cf. section 2.4) because it showed how to supply slightly better numerical solutions with respect to the Langmuir isotherm. As a general consideration, the adopted kinetic model provides a satisfying interpretation of kinetic adsorption data as testified by the good agreement between experimental and numerical tb values (cf. Table 3). Noteworthy, the computed fixed-bed Péclet number was higher

Figure 4. CO2 adsorption isotherms at 293 K for F600−900 and F900− 1200 adsorbents.

CO2 adsorption isotherms at 293 K obtained for F600−900 and F900−1200 activated carbons. The plots show that, as expected, ωeq increases with Peq with a nonlinear tendency as a consequence of the relatively high CO2 concentration in the gas stream. Moreover, the ωeq value corresponding to a 15% CO2 initial concentration and representative of a typical flue gas composition is about 0.7 mol kg−1 for both adsorbents. This result is in good accordance with those reported in the literature for activated carbons tested under similar experimental conditions.13,26 In addition, experimental results practically coincide for the two investigated solids in all the CO2 concentration ranges explored; this testifies that the difference in particle sizes for F600−900 and F900−1200 does not affect the solid capture performances under equilibrium conditions. In this context, on the basis of the sorbents properties reported in Table 1, one would have expected higher CO2 capture capacity for F600−900 sample because of its higher surface area and micropore volume (the chemical compositions are identical for the two activated carbons). Nevertheless, as highlighted by Wahby et al.,22 the presence of narrow micropores seems to be a key factor in determining CO2 adsorption, because in this class of pores, the overlapping potential produces a more effective packing of CO2 molecules. As a consequence, the same value of Vn derived for F600−900 and F900−1200 (cf. Table 1) is likely to be responsible for their equivalent CO2 capture performance under equilibrium conditions. Thermodynamic parameters obtained from the application of Langmuir and Freundlich models to raw equilibrium adsorption data are listed in Table 2. In general, it is possible Table 2. Langmuir and Freundlich Parameters Obtained for CO2 Adsorption at 293 K onto F600−900 and F900−1200 Adsorbents Langmuir

Freundlich

KL, bar−1 ωmax, mol kg−1 R2, − KF, mol kg−1 bar−1/n 1/n, − R2, −

F600−900

F900−1200

2.86 2.43 0.999 2.70 0.71 0.996

2.94 2.32 0.997 2.58 0.70 0.998

to observe that both adsorption models determine an excellent fitting of adsorption data, as witnessed by the high R2 values 12187



Article

Figure 5. Experimental breakthrough curves obtained for F600−900 and F900−1200 at 293 K for different CO2 initial volumetric concentration in the feed: (a) 1%, (b) 3%, (c) 5%, (d) 7%, (e) 9%, (f) 11%, (g) 13%, (h) 15%.

particle size on CO2 mass-transfer resistance have been reported in the literature for different adsorbents.31,41 For example, Ding and Alpay31 observed an increase in the masstransfer coefficient by a factor 10 for CO2 adsorption at 673 K and under wet conditions onto crushed hydrotalcite (mean particle diameter 0.5 mm) with respect to the parent material (equivalent particle size 2.75 mm). Finally, Soares et al.41 reported that intraparticle diffusion resistance was double for a similar increase in particle size when studying CO2 adsorption onto Brazilian coals at 303 K and 1 bar adopting a 19.9/80.1% CO2/He gas mixture.

than 100; thus it was possible to consider a plug-flow model for the examined gas−solid systems.40 A comparison between mass-transfer resistances highlights that intraparticle diffusion mechanism represents the rate-determining step of the adsorption process for both F600−900 and F900−1200 adsorbents. In fact, Ωp is 3 orders of magnitude greater than Ωext. Moreover, numerical analysis provides higher masstransfer coefficients for F600−900, thus confirming observations inferred from experimental breakthrough curves. For example, Ωp is approximately double in the case of F900− 1200 adsorbent. Similar findings concerning the effect of 12188



Article

In conclusion, this work highlighted the need of a preliminary particle size screening, for a granulometrically heterogeneous adsorbent, to assess the effect of the different microstructural solid properties on CO2 capture performances, mostly under dynamic conditions. This step allows an optimization of the adsorbent utilization also in light of a proper functionalization treatment aimed at enhancing its adsorptive properties toward CO2 (e.g., impregnation with basic-amine compounds).

Table 3. Fixed Bed Properties and Main Fluid Dynamic/Kinetic Parameters Obtained from Mathematical Modeling of 15% CO2 Breakthrough Curves for F600−900 and F900−1200 Adsorbents m, kg L, m ε, − ρp, kg m−3 Pe, − Ωext, s Ωp, s tbexp, s tbmod, s

F600−900

F900−1200

1.5 × 10−2 9.2 × 10−2 0.47 519 109 1.4 × 10−3 1.7 47 43

1.5 × 10−2 9.5 × 10−2 0.52 503 101 2.4 × 10−3 3.3 44 40

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +39-081-674029. Fax: +39-081-674090. E-mail: fabio. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Massimo Urciuolo (IRC-CNR) and Dr. Ana Maria Silvestre-Albero (LMA) for their help in adsorbents characterization. The experimental work of Mr. Raffaele Pagnano (UNINA) is also acknowledged.

■

NOMENCLATURE Freundlich heterogeneity parameter [−] CO2 concentration in the bulk gas phase [mol m−3] CO2 concentration in the gas phase at the fluid-particle interface [mol m−3] Cin CO2 concentration in the gas phase at the fixed-bed inlet [mol m−3] Dax axial dispersion coefficient [m2 s−1] DCO2−N2 CO2 molecular diffusivity in the gas phase [m2 s−1] Dp CO2 intraparticle diffusivity [m2 s−1] dS mean Sauter particle diameter [m] KF Freundlich affinity constant [mol kg−1 bar−1/n] KL Langmuir equilibrium constant [bar−1] kext CO2 fluid film mass-transfer coefficient [m s−1] L packed-bed length [m] MCO2 CO2 molecular weight [kg mol−1] m sorbent amount [kg] P/P0 relative pressure [−] Peq CO2 equilibrium partial pressure in the gas phase [bar] Pe fixed-bed Péclet number [−] QinCO2 CO2 volumetric flow rate at the fixed-bed inlet [m3 s−1] Qout CO2 volumetric flow rate at the fixed-bed outlet [m3 CO2 s−1] 2 R determination coefficient [−] RS mean Sauter particle radius [m] r radial particle coordinate [m] Re Reynolds number [−] SBET apparent surface area [m2 g−1] Sc Schmidt number [−] Sh Sherwood number [−] t time [s] tb breakpoint time [s] tbexp experimental breakpoint time [s] tbmod numerical breakpoint time [s] teq equilibrium adsorption time for which QinCO2 = Qout CO2 [s] −1 u gas superficial velocity [m s ] V0 micropore volume [cm3 g−1] Vmeso mesopore volume [cm3 g−1] Vn narrow micropore volume [cm3 g−1] Vt total pore volume [cm3 g−1] 1/n C C*

Figure 6. Comparison between experimental (symbols) and theoretical (lines) breakthrough curves obtained for F600−900 and F900−1200 at 293 K for a 15% CO2 gas stream.

4. CONCLUSIONS In this work two particle size classes of commercial activated carbon Filtrasorb 400, namely 600−900 μm (sample F600−900) and 900−1200 μm (sample F900−1200), were investigated as adsorbents for CO2 capture at 293 K from model flue gas streams. F600−900 and F900−1200 displayed identical chemical compositions, whereas N2/CO2 porosimetric analyses highlighted a higher pore volume and the presence of a broader micropore size distribution with wider micropores for F600−900 class. CO2 thermodynamic adsorption tests at 293 K highlighted equivalent capture performances under equilibrium conditions for both investigated adsorbents, due to the same value of the volume of narrow micropores, the latter considered in the literature to play a predominant role in CO2 capture. Adsorption isotherms at 293 K have been interpreted in light of Langmuir and Freundlich models, and both of them provided an excellent fitting of equilibrium adsorption data. Breakthrough curves obtained for 1−15% CO2 volumetric concentrations in the feed showed longer breakpoint times and faster pollutant capture kinetics for F600−900, likely ascribable to faster diffusion rates of CO2 in wider micropores observed for this adsorbent. Kinetic experimental evidence were also corroborated by mathematical modeling of breakthrough curves for a typical 15% CO2 flue gas stream, which supplied higher mass-transfer coefficients for F600−900 with respect to F900−1200. Numerical analysis of adsorption kinetics allowed identifying intraparticle diffusion as the rate-determining step of the adsorption process. 12189


Industrial & Engineering Chemistry Research VadsSTP z

Article

specific N2 adsorbed volume @ STP [cm3 g−1] fixed-bed axial coordinate [m]

(17) Vargas, D. P.; Giraldo, L.; Erto, A.; Moreno-Piraján, J. C. Chemical Modification of Activated Carbon Monoliths for CO2 Adsorption. J. Therm. Anal. Calorim., in press. (18) Balsamo, M.; Budinova, T.; Erto, A.; Lancia, A.; Petrova, B.; Petrov, N.; Tsyntsarski, B. CO2 Adsorption onto Synthetic Activated Carbon: Kinetic, Thermodynamic and Regeneration Studies. Sep. Purif. Technol. 2013, 116, 214. (19) Przepiórski, J.; Czyżewski, A.; Pietrzak, R.; Morawski, A. W. MgO/CaO-Loaded Activated Carbon for Carbon Dioxide Capture: Practical Aspects of Use. Ind. Eng. Chem. Res. 2013, 52, 6669. (20) Marsh, H.; Rodrι ́guez-Reinoso, F. Activated Carbon; Elsevier Science & Technology Books: Amsterdam, 2006. (21) Wahby, A.; Ramos-Fernández, J. M.; Martι ́nez-Escandell, M.; Sepúlveda-Escribano, A.; Silvestre Albero, J.; Rodrι ́guez-Reinoso, F. High-Surface-Area Carbon Molecular Sieves for Selective CO 2 Adsorption. ChemSusChem 2010, 3, 974. (22) Wahby, A.; Silvestre-Albero, J.; Sepúlveda-Escribano, A.; Rodríguez-Reinoso, F. CO2 Adsorption on Carbon Molecular Sieves. Microporous Mesoporous Mater. 2012, 164, 280. (23) Caglayan, B. S.; Aksoylu, A. E. CO2 Adsorption on Chemically Modified Activated Carbon. J. Hazard. Mater. 2013, 252−253, 19. (24) Gomes, V. G.; Yee, K. W. K. Pressure Swing Adsorption for Carbon Dioxide Sequestration from Exhaust Gases. Sep. Purif. Technol. 2002, 28, 161. (25) Tlili, N.; Grévillot, G.; Vallières, C. Carbon Dioxide Capture and Recovery by means of TSA and/or VSA. Int. J. Greenhouse Gas Control 2009, 3, 519. (26) Plaza, M. G.; Pevida, C.; Pis, J. J.; Rubiera, F. Evaluation of the Cyclic Capacity of Low-Cost Adsorbent for Post-Combustion CO2 capture. Energy Procedia 2011, 4, 1228. (27) Khalil, S. H.; Aroua, M. K.; Daud, W. M. A. W. Study on the Improvement of the Capacity of Amine-Impregnated Commercial Activated Carbon Beds for CO2 Adsorbing. Chem. Eng. J. 2012, 183, 15. (28) Delgado, J. M. P. Q. A Critical Review of Dispersion in Packed Beds. Heat Mass Transfer 2006, 42, 279. (29) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (30) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Series on Chemical Engineering; Vol. 2, Imperial College Press: London, 1998. (31) Ding, Y.; Alpay, E. Equilibria and Kinetics of CO2 Adsorption on Hydrotalcite Adsorbent. Chem. Eng. Sci. 2000, 55, 3461. (32) Delgado, J. A.; Uguina, M. A.; Sotelo, J. L.; Ruíz, B. Fixed-Bed Adsorption of Carbon Dioxide-Helium, Nitrogen-Helium and Carbon Dioxide-Nitrogen Mixtures onto Silicalite Pellets. Sep. Purif. Technol. 2006, 49, 91. (33) Serna-Guerrero, R.; Sayari, A. Modeling Adsorption of CO2 on Amine-Functionalized Mesoporous Silica. 2: Kinetics and Breakthrough Curves. Chem. Eng. J. 2010, 161, 182. (34) Shen, C.; Grande, C. A.; Li, P.; Yu, J.; Rodrigues, A. E. Adsorption Equilibria and Kinetics of CO2 and N2 on Activated Carbon Beads. Chem. Eng. J. 2010, 160, 398. (35) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997. (36) Balsamo, M.; Di Natale, F.; Erto, A.; Lancia, A.; Montagnaro, F.; Santoro, L. Gasification of Coal Combustion Ash for Its Reuse as Adsorbent. Fuel 2013, 106, 147. (37) Patrick, J. W. Porosity in Carbons: Characterization and Applications; Halsted Press: Sydney, 1995. (38) Morlay, C.; Joly, J. P. Contribution to the Textural Characterization of Filtrasorb 400 and Other Commercial Activated Carbons Commonly Used for Water Treatment. J. Porous Mater. 2010, 17, 535. (39) Krutyeva, M.; Grinberg, F.; Furtado, F.; Galvosas, P.; Kärger, J.; Silvestre-Albero, A.; Sepulveda-Escribano, A.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. Characterization of Carbon Materials with the Help of NMR Methods. Microporous Mesoporous Mater. 2009, 120, 91.

Greek Symbols

ε μ ρCO2 ρg ρp Ωext Ωp ω ω̅ ωeq ωmax

■

fixed-bed voidage fraction [−] gas phase dynamic viscosity [kg m−1 s−1] CO2 density [kg m−3] gas phase density [kg m−3] adsorbent particle density [kg m−3] film diffusion resistance [s] intraparticle diffusion resistance [s] adsorption capacity in the adsorbent particle [mol kg−1] adsorption capacity averaged over an adsorbent particle [mol kg−1] equilibrium adsorption capacity [mol kg−1] Langmuir monolayer adsorption capacity [mol kg−1]

REFERENCES

(1) Metz, B.; Davidson, O.; de Coninck, H.; Loos, M.; Meyer, L. Intergovernmental Panel on Climate Change. Carbon Dioxide Capture and Storage; Cambridge, MA, 2005. (2) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 Capture Technology-The U.S. Department of Energy’s Carbon Sequestration Program. Int. J. Greenhouse Gas Control 2008, 2, 9. (3) Li, F.; Fan, L. S. Clean Coal Conversion Processes−Progress and Challenges. Energy Environ. Sci. 2008, 1, 248. (4) Lee, Z. H.; Lee, K. T.; Bhatia, S.; Mohamed, A. R. Post-Combustion Carbon Dioxide Capture: Evolution Towards Utilization of Nanomaterials. Renewable Sustainable Energy Rev. 2012, 16, 2599. (5) Uyanga, I. J.; Idem, R. O. Studies of SO2- and O2-Induced Degradation of Aqueous MEA during CO2 Capture from Power Plant Flue Gas Streams. Ind. Eng. Chem. Res. 2007, 46, 2558. (6) Brúder, P.; Svendsen, H. F. Capacity and Kinetics of Solvents for Post-Combustion CO2 Capture. Energy Procedia 2012, 23, 45. (7) Kittel, J.; Idem, R.; Gelowitz, D.; Tontiwachwuthikul, P.; Parrain, G.; Bonneau, A. Corrosion in MEA Units for CO2 Capture: Pilot Plant Studies. Energy Procedia 2009, 1, 791. (8) Abanades, J. C.; Rubin, E. S.; Anthony, E. J. Sorbent Cost and Performance in CO2 Capture Systems. Ind. Eng. Chem. Res. 2004, 43, 3462. (9) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2, 796. (10) Balsamo, M.; Di Natale, F.; Erto, A.; Lancia, A.; Montagnaro, F.; Santoro, L. Arsenate Removal from Synthetic Wastewater by Adsorption onto Fly Ash. Desalination 2010, 263, 58. (11) Itskos, G.; Koukouzas, N.; Vasilatos, C.; Megremi, I.; Moutsatsou, A. Comparative Uptake Study of Toxic Elements From Aqueous Media by the Different Particle-Size-Fractions of Fly Ash. J. Hazard. Mater. 2010, 183, 787. (12) Balsamo, M.; Di Natale, F.; Erto, A.; Lancia, A.; Montagnaro, F.; Santoro, L. Cadmium Adsorption by Coal Combustion Ashes-Based Sorbents−Relationship between Sorbent Properties and Adsorption Capacity. J. Hazard. Mater. 2011, 187, 371. (13) Sayari, A.; Belmabkhout, Y.; Serna-Guerrero, R. Flue Gas Treatment via CO2 Adsorption. Chem. Eng. J. 2011, 171, 760. (14) Balsamo, M.; Di Natale, F.; Erto, A.; Lancia, A.; Montagnaro, F.; Santoro, L. Reuse of Coal Combustion Ash as Sorbent: the Effect of Gasification Treatments. Combust. Sci. Technol. 2012, 184, 956. (15) Balsamo, M.; Di Natale, F.; Erto, A.; Lancia, A.; Montagnaro, F.; Santoro, L. Steam- and Carbon Dioxide-Gasification of Coal Combustion Ash for Liquid Phase Cadmium Removal by Adsorption. Chem. Eng. J. 2012, 207−208, 66. (16) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-Combustion CO2 Capture Using Solid Sorbents: a Review. Ind. Eng. Chem. Res. 2012, 51, 1438. 12190



Article

(40) Inglezakis, V. J.; Poulopoulos, S. G. Adsorption, Ion Exchange and Catalysis. Design of Operations and Environmental Application; Elsevier: Amsterdam, 2006. (41) Soares, J. L.; Oberziner, A. L. B.; José, H. J.; Rodrigues, A. E.; Moreira, R. F. P. M. Carbon Dioxide Adsorption in Brazilian Coals. Energy Fuels 2007, 21, 209.

12191


Highlighting the Role of Activated Carbon Particle Size on CO2

Recommend Documents