281750 Kinetics and Modeling of CO2 Adsorption On Amine Functionalized Mesoporous SBA-15 Sorbents
Kinetics and Modeling of CO2 Adsorption on Amine Functionalized Mesoporous SBA-15 Sorbents
Arunkumar Samanta1, An Zhao1, Partha Sarkar2, Rajender Gupta1
1Department of Chemical and Materials Engineering, University of Alberta
9107 - 116 St, ECERF, AB, T6G 2V4, Canada, Email:firstname.lastname@example.org, Tel: 780-492-6861, Fax: 780-492-2881
2 Environmental & Carbon Management Division, Alberta Innovates - Technology Futures, Edmonton, AB, Canada,
Recently, adsorption processes using functionalized ordered mesoprous silica sorbents have shown a great potential for post-combustion CO2 capture from flue gas compared to the other separation techniques. Ordered mesoporous silicas generally provide excellent structural properties as supports, such as large pore volume, pore size and surface area. In addition to this, numerous active sites disperse highly on the inner pore surface which benefit and promote the distribution of amine moieties on the surface. The immobilization of amino groups anchored to the surface of solid support by means of physical impregnation or covalent modification via silane chemistry offers stable interaction with acidic CO2 molecules, meanwhile largely avoids the corrosion and toxic problems.
In this work an experimental and theoretical investigation on the adsorption of CO2 onto amine- functionalized mesoporous SBA-15. The adsorption of CO2 on the functionalized sorbents have been measured by thermogravimetric method over the CO2 partial pressure range of 10-100 kPa and temperature range of 303 – 373 K under atmospheric pressure. The functionalization of SBA-15 silicas with tetraethylenepentamine (TEPA) has been achieved using conventional wet impregnation technique. The structural properties of the sorbents have been characterized by nitrogen adsorption/desorption, SAXS, SEM, TEM and FTIR techniques. Thermal swing adsorption cycles over a range of temperatures and time in a simulated flue gas were also explored. Different chemisorption kinetic model has been developed to analyze the experimental data. The model is validated with the experimental results of isothermal adsorption measurements of CO2 on SBA-15/TEPA.
2.1 Material Synthesis and Characterization
The mesoporous SBA-15 material used in this work was synthesized using amphiphilic triblock copolymer Pluronic P123 (MWavg = 5800, Aldrich) as the organic structure-directing agent, tetraethyl orthosilicate (TEOS, Aldrich) as silica source and HCl as pH controlling agent. The resulting SBA-15 materials, after drying, calcination and subsequent impregnation, were characterized by N2 adsorption/desorption, SAXS, SEM, TEM and FTIR.
2.2 CO2 Adsorption and Regeneration Study
CO2 adsorption/desorption measurements were performed on a thermogravimetric analyzer (Q500, TA Instruments). Pure CO2 (99.99%) or CO2/ N2 at 1 atm was used for the adsorption measurements runs and N2 was used as purging gas for CO2 desorption. In a typical adsorption run, about 10 mg of the sorbent were placed in platinum pan. After the sorbent was heated to 105 °C in a N2 stream with a flow rate of about 100 cm3. min-1 for about 30 min to remove all moisture and CO2 adsorbed from air, the temperature was decreased to desired adsorption temperature. The gas was then switched from N2 to CO2 and the temperature was kept constants at desired adsorption temperature for about 1 h. The CO2 capturing capacity of the solid sorbent was calculated in mmol.g-1 dry sorbent from the weight gain of the sample in the adsorption process.
3. Results and Discussion
The N2 adsorption isotherms of calcined SBA-15 shows typical type IV isotherm indicative of defined mesoporosity in the framework. For calcined SBA-15, the BET surface area, mesopore area and mesopore volume are about 1099 m2.g-1, 923 m2.g-1 and 1.78 cm3.g-1, respectively. The pore size (in the range of 10-30 nm) was estimated using BJH method. Five distinct reflective peaks are identified from the SAXS diffraction spectra of SBA-15 and SBA-15/TEPA. The estimated d-spacing (d100) is in the range of 9.7 -10.3 nm. The morphology of the mesoporous SBA-15 support studied using SEM and TEM techniques. The SBA-15 is observed to contain rope like material arranged in bundle of about 4.3 µm diameter and length of 35 µm. Besides, FTIR spectroscopy of SBA-15/TEPA provided clear evidence for amine impregnation.
3.2 CO2 Adsorption Study
CO2 adsorption capacities of the amine impregnated solid sorbent supports, such as Norit AC, Sigma AC (C 5510), SBA-15, and MCM-41 has been determined at 303, 323, 348 and 373 K in pure CO2 and various partial pressures of CO2 (in N2). Typical CO2 adsorption capacity results are shown and compared in Figure 1. From the preliminary screening study for TEPA and PEI impregnated sorbents, it has been found that SBA-15/TEPA outperforms all other sorbents under the same conditions. In particular, SBA-15/TEPA sorbents shows better CO2 adsorption capacity than the sorbent loaded with same amount of PEI. SBA-15/TEPA exhibits the highest CO2 adsorption capacity and reached an CO2 uptake value as high as 4.51 mmol-CO2/g-dry sorbent within few minutes of adsorption test at 348 K and pure CO2 environment. Amine modified activated carbon does not show satisfactory performance on adsorption capacity because of the limitations such as relatively lower pore volume and surface area.
To study the influence of adsorption temperature on TEPA impregnated SBA-15 sorbents, adsorption experiments were carried out at 303, 323, 348 and 373 K. When the temperature increases from 303 to 348 K, there is a large difference observed in adsorption capacity that varies from 2.73 to 4.51 mmol.g-1. One possible reason is that higher temperature promotes the adsorption reaction and increases the accessibility of CO2 to available active sites on the surface of TEPA. On the other hand, all the sorbents exhibit a lower adsorption capacity at low CO2 partial pressure. Preliminary test with ~2.0 (vol) % moisture in pure CO2 shows CO2 adsorption capacity of about 5.05 mmol.g-1 indicating moisture favors in CO2 adsorption.
A series of impregnated sorbent using calcined SBA-15 containing 50- 70 wt% TEPA were also prepared to investigate the effect of amine loading on adsorption capacity. It is observed that the adsorption capacity increases from 3.57 mmol.g-1 to 4.60 mmol.g-1 with increasing the amine loading from 50 wt% to 70 wt%. However, the enhancement of adsorption capacity with respect to amine efficiency does not follow similar trend. For example, SBA-15/TEPA 60 exhibits the highest amine efficiency 0.290, while SBA-15/TEPA70, which presents the highest adsorption capacity, has relatively lower amine efficiency value of 0.248.
About ten cycles of adsorption-desorption were performed to study the cyclic adsorption-desorption performance of the SBA-15/TEPA60 sorbent using temperature swing process. From the cyclic adsorpion-desorpiton performance, it is observed that after ten cylces of adsorption-desorption, the sorbent shows a gradual decrease in capacity. This is probably due to the continuous degdation of TEPA during the repeated adsorption-desorption processes.
3.3 CO2 Adsorption Kinetics
A fast adsorption kinetics have been found with more than 90% of the total CO2 uptake on TEPA-SBA-15 sorbents occurring within the first few minutes (< 5mins ) of adsorption. All SBA-15/TEPA show a sharp increase of adsorption during the initial adsorption stage. Three different kinetic models expressed by Eq.(1), Eq. (2) and Eq.(3), have been used to analyze the experimental CO2 uptake data:
where qe and qt are the sorption capacity at equilibrium and at time t, respectively; k, kn, n and m are the model constants. Figure 2 shows the experimental results of isothermal adsorption measurements of CO2 on functionalized SBA-15 sorbents at 348 K and the corresponding values predicted by various kinetic models. It has been observed that kinetic model described by Eq. (3) is in good agreement with the experimental data.
This work presents a theoretical and experimental investigation on the adsorption of CO2 onto amine- functionalized mesoporous SBA-15. The CO2 sorption/desorption experiments performed by thermogravimetric method revealed that the well-dispersed TEPA inside SBA-15 exhibited a CO2 sorption capacities as high as 4.51 mmol.g-1 for the sorbent SBA-15/TEPA60 at 348 K. The textural properties of the raw materials and in particular, the mesopore content seems to be the dominant factor for creating a good dispersion of the amines into the pore channels of the silica support. The CO2 capture kinetics is found to be fast and reached 90% of the total capacities within the first ten minutes. Different chemisorption kinetic model has been developed to analyze the experimental data. The model is validated with the experimental results of isothermal adsorption measurements of CO2 on functionalized SBA-15 sorbents by thermogravimetric method. Good agreement between the model results and experimental results indicates that a general kinetic model described by Eq. (3) can effectively represent the dynamics of CO2 mass transfer rates on TEPA functionalized SBA-15 sorbents for wide range of operating conditions. The cyclic CO2 adsorption-desorption revealed that the TEPA impregnated SBA-15 sorbents exhibited good cyclic stability. The results suggest that mesoporous SBA-15 impregnated with TEPA shows a great potential as adsorbents for post-combustion CO2 capture.
The financial support of the Canadian Centre for Clean Coal/Carbon and Mineral Processing Technology and Carbon Management Canada is acknowledged.