Article pubs.acs.org/IECR
Equilibrium and Dynamic CO2 Adsorption on Activated Carbon Honeycomb Monoliths D. P. Vargas,† M. Balsamo,‡ L. Giraldo,† A. Erto,*,‡ A. Lancia,‡ and J. C. Moreno-Piraján§ †
Facultad de Ciencias, Departamento de Química, Universidad Nacional de Colombia, Avenida Carrera 30 No. 45-03, Bogotá, Colombia ‡ Dipartimento di Ingegneria Chimica dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzelle Tecchio, 80, 80125 Napoli, Italy § Facultad de Ciencias, Departamento de Química, Universidad de los Andes, Carrera 1 No. 18 A 10, Bogotá, Colombia ABSTRACT: In this work, CO2 adsorption tests from simulated flue gas are carried out in a laboratory-scale fixed-bed reactor, at different temperatures (303 and 353 K) and CO2 concentrations (3−25%), in order to investigate both the kinetics and thermodynamics. To this purpose, two different activated carbon (AC) monoliths were synthesized starting from an African Palm stone (Elaeis guineensis), activated either with H3PO4 (48% w/v) or with a combination of H3PO4 (32% w/v) + CaCl2 (2% w/ v), respectively. In order to increase the affinity toward CO2, the AC monoliths were subjected to the same surface modification postprocess, conducted with a 30% (w/w) aqueous ammonia solution. The textural characterization of the AC monoliths was carried out by N2 and pure CO2 adsorption at 77 and 273 K, respectively, allowing determination of the micro- and mesopore volumes as well as pore-size distributions. The adsorption tests show a maximum CO2 adsorption capacity for the sample with the greatest ultramicropore volume, while the adsorption rate increases in the presence of mesopores and for higher temperature. Moreover, dedicated regeneration studies demonstrate that both of the AC monoliths can be fully regenerated at each investigated adsorption temperature and their CO2 adsorption capacity remains almost constant in five consecutive cycles of adsorption−desorption. It can be concluded that AC monoliths can be a good alternative to granular or powdered AC for CO2 capture in fixed-bed adsorption columns.
1. INTRODUCTION
Currently, the most mature CO2 capture technology among the postcombustion options is chemical absorption, with a monoethanolamine (MEA) aqueous solution being the most widely used solvent. However, this system is characterized by several drawbacks such as high volatility, thermal and chemical instability of MEA, severe corrosion action on process equipment, and a marked energy requirement for solvent regeneration.6 As an alternative, CO2 adsorption on porous solids has received great attention for reducing the energy penalty of the capture step.7,8 Many different solids have been proposed like mesoporous silica, activated carbons (ACs), zeolites, hydroxy metal carbonates, metal oxides, metal−organic frameworks, etc.9−11 but the quest for outstanding and cost-effective sorbents is still an emergent research area.12 AC is often chosen as an adsorbing material, considering its economic advantages over other materials and, in many cases, for the comparable adsorption capacity.13−15 Traditionally, the porosity and surface area have been considered as the main controlling parameters that define the performances of an AC,14,16,17 but recent literature studies demonstrate that the
Carbon dioxide (CO2) is among the major greenhouse gases causing global warming. The increase in CO2 emissions recorded in the last decades is mainly related to power generation, transportation, and industrial processes. Today, about 85% of the global energy demand is supplied by fossil fuels such as oil, natural gas, coal, etc.; in the next future, these are expected to still play a significant role in the global energy economy, mainly because of their abundance and the everincreasing world population.1,2 It is estimated that in 2009 the global CO2 emissions derived from fossil fuels amounted to about 30 Gt.2 The need for a drastic reduction of CO2 emissions has gained the attention of both the public lay and researchers, and concerted actions have become necessary in order to stabilize the atmospheric level of CO2. CO2 capture and storage (CCS) is unanimously considered as one of the most recommended strategies for achieving massive reductions of the growing emission levels of CO2 from large point sources. In particular, postcombustion CO2 capture can contribute in the short-tomedium term because it eliminates the need for substantial modifications of the combustion processes.3 Thus, it can be applied to both new and existing stationary fossil-fuel-fired power plants, at relatively low technology risk. It has been reported that about two-thirds of the average costs of CCS lie in the CO2 capture step.4,5 Therefore, the development of efficient and cost-effective CO2 capture technologies is one of the highest priorities in the field of CCS. © XXXX American Chemical Society
Special Issue: International Conference on Carbon Dioxide Utilization 2015 Received: September 1, 2015 Revised: October 20, 2015 Accepted: October 26, 2015
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DOI: 10.1021/acs.iecr.5b03234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
2. MATERIALS AND METHODS 2.1. Monolith Preparation. The AC monoliths were prepared using African Palm stones as lignocellulosic precursors, previously ground to 38 μm particle size. The activation process was conducted in an aqueous solution of either H3PO4 (48% w/v) (MP48 sample) or a combination of H3PO4 (32% w/v) + CaCl2 (2% w/v) (MCa2 sample), with a constant proportion of 1 × 10−3 kg of precursor material per 2 × 10−3 L of solution. The monoliths were prepared using a compacting process in which the impregnated precursor was put in a uniaxial press and subjected to a pressure of 31 MPa, at 423 K for 30 s. The obtained monoliths have an outer diameter of 1.5 × 10−2 m and a height of 8 × 10−3 m and are traversed longitudinally by six parallel channels, each with an inner diameter of 3 × 10−3 m. A subsequent carbonization process was carried out in order to further develop the porosity of the sorbents. After the synthesis, the MP48 and MCa2 samples were subjected to the same functionalization process aimed at inserting nitrogen groups on their surface, conducted in an aqueous phase with a 30% (w/w) aqueous ammonia solution, obtaining the samples named MP48FAL and MCa2FAL, respectively. All of the experimental conditions used for the monolith synthesis and functionalization are reported in previously published works34,35 and summarized in Table 1.
surface chemistry plays an important role as well, also for CO2 adsorption.9,18,19 For this reason, an appropriate postsynthesis surface modification can be an interesting route to further improve the solid performances, determining specific interactions between the adsorbent and adsorbate. As an example, the introduction of nitrogen functionalities may be achieved either by impregnating the surface with appropriate chemicals or by incorporating nitrogen into the carbon structure.20−23 Many researchers have published works on the preparation of AC-based adsorbents with excellent capture performances for pure CO2.11,13,21,24−26 Conversely, few studies are performed in simulated flue-gas streams (i.e., PCO2 < 0.15 bar) and/or in dynamic conditions, and even fewer studies are available in which the adsorption of CO2 is investigated simultaneously from simulated flue gas, in dynamic systems, and as a function of the main operational parameters (e.g., gas temperature, CO2 concentration, etc.).17,27,28 In addition, ACs are typically obtained either in a finely powdered form or as a granular material to make them suitable for handling in special devices. However, in spite of the widespread utilization of granular/powdered adsorbents, there are some drawbacks when applied to real scale, such as high pressure drop, channeling, gas bypassing in packed beds, and attrition due to the granular material and to particle entrainment in fluidized beds.29,30 As an alternative, to overcome these drawbacks, the production of AC monoliths is gaining crescent interest, also for CO2 capture.31 An appropriate synthesis procedure assures minimal or negligible modification of the textural properties with respect to the granular homologous adsorbents. However, many issues should still be addressed in this field of investigation such as the synthesis procedure, type of binder (if any), and postfunctionalization treatment and the effect of all of these factors on the porous properties of the final adsorbent.32,33 In previous works of our research group, the production of monoliths and the definition of the optimal chemical and textural characteristics for CO2 adsorption were carried out.19,34,35 However, CO2 equilibrium and dynamic adsorption tests should be made in order to define the real potentiality of the AC monoliths, in experimental conditions typical of flue gas. In this work, two AC monoliths were synthesized for CO2 adsorption applications, starting from the same lignocellulosic precursor, an African Palm stone (Elaeis guineensis). To this aim, two different activating aqueous solutions were used, i.e., either H3PO4 (48% w/v) or H3PO4 (32% w/v) + CaCl2 (2% w/v). Subsequently, the AC monoliths were subjected to the same surface modification postprocess, conducted with a 30% (w/w) aqueous ammonia solution, in order to increase the CO2 adsorption capacity. Both of the AC monoliths produced were characterized by adsorption of N2 at 77 K and pure CO2 at 273 K. Boehm titration analysis and pHPZC calculation were performed for a chemical characterization of the monolith samples. CO2 adsorption tests from simulated flue gas (i.e., PCO2 < 0.15 bar, balance N2) were carried out in a laboratory-scale plant based on a fixed-bed column, at two temperature levels (i.e., 303 and 353 K). Kinetic and thermodynamic aspects of CO2 adsorption were discussed, based on analysis of the physical and chemical properties of the monoliths. Finally, the possibility of reutilization of the AC monoliths within consecutive cycles of adsorption−desorption was assessed, in order to define their actual potentiality for postcombustion CO2 capture.
Table 1. Experimental Conditions for Synthesis and Functionalization of the AC Monoliths treatment step impregnation drying carbonization
washing functionalization washing drying storage
MP48FAL
MCa2FAL
H3PO4 (48% w/ v), 358 K, 2 h
H3PO4 (32% w/w) + CaCl2 (2% w/v), 358 K, 7 h 393 K, ∼8 h 723 K, 2 h (1) 1073 K, (2) 873 K, 2 h 6h N2: 80 mL min−1 CO2: 100 N2: 80 mL min−1 mL min−1 heating rate: 1 heating rate: heating rate: K min−1 3 K min−1 3 K min−1 hot distilled water (1) hot 0.01 (2) hot distilled until neutral pH M HCl water until neutral pH NH3(aq) solution (30% w/w), 353 K, 24 h hot distilled water until neutral pH 393 K, 6 h plastic containers, plastic containers, N2 atmosphere N2 atmosphere
2.2. Monolith Characterization. The textural properties of the synthesized AC monoliths were determined by N2 adsorption at 77 K and pure CO2 adsorption at 273 K, in a volumetric system (Quantachrome, Autosorb 3-B). Before the adsorption experiments, the samples were submitted to an outgassing treatment at 423 K for 24 h in order to remove the humidity. The main microstructural parameters of the AC monoliths were obtained from the mathematical processing of N2 and CO2 adsorption isotherms, according to the models commonly applied in the literature.14 In particular, the “apparent” surface area (SBET) was obtained using the Brunauer−Emmett−Teller (BET) method. The volume of the micropores, V0, and the volume of the narrow micropores (Vn; pores
