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Mathematical Modeling and Experimental Breakthrough Curves of Carbon Dioxide Adsorption on Metal Organic Framework CPM-5 Rana Sabouni, Hossein Kazemian, and Sohrab Rohani Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es401276r • Publication Date (Web): 26 Jul 2013 Downloaded from http://pubs.acs.org on July 28, 2013
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The effect of feed flow rate on the CO2 breakthrough time for, a) 12.5 % CO2 at 298 K, b) 12.5 % CO2 at 318 K, c) 25 % CO2 at 298 K , and d) 25 % CO2 at 318 K. 188x134mm (300 x 300 DPI)
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The effect of temperature on the CO2 breakthrough time for, a) 12.5 % CO2 and Ft= 8 mL/min, b) 25 % CO2 and Ft= 8 mL/min, c) 12.5 % CO2 and Ft= 32 mL/min, and d) 25 % CO2 and Ft= 32 mL/min. 197x142mm (300 x 300 DPI)
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The effect of CO2 feed concentration on the CO2 breakthrough time for, a) T= 298 K and Ft= 8 mL/min, b) T= 298 K and Ft= 32 mL/min, c) T= 318 K and Ft= 8 mL/min, and d), T= 318 K and Ft= 32 mL/min.
179x125mm (300 x 300 DPI)
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Comparing of CO2 breakthrough curves on the regenerated CPM-5 and its fresh counterpart at the same adsorption condition (CPM-5 is fully regenerated after subjecting to N2 flow of 100 mL/min at 373 K for 1 h). 82x52mm (300 x 300 DPI)
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TOC Art 264x318mm (300 x 300 DPI)
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Mathematical Modeling and Experimental Breakthrough Curves of Carbon
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Dioxide Adsorption on Metal Organic Framework CPM-5
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Rana Sabouni, Hossein Kazemian, Sohrab Rohani*
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Department of Chemical and Biochemical Engineering, Western University, London, Ontario, Canada
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N6A5B9
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Abstract
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It is essential to capture carbon dioxide from flue gas because it is considered one of the main
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causes of global warming. Several materials and different methods have been reported for the
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CO2 capturing including adsorption onto zeolites, porous membranes as well as absorption in
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amine solutions. All such methods require high energy input and high cost. New classes of
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porous material called Metal Organic Frameworks (MOFs) exhibited excellent performance
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in extracting carbon dioxide from a gas mixture. In this study, the breakthrough curves for
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the adsorption of carbon dioxide on CPM-5 (Crystalline Porous Materials) were obtained
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experimentally and theoretically using a laboratory scale fixed bed column at different
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experimental conditions such as feed flow rate, adsorption temperature and feed
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concentration. It was found that the CPM-5 has a dynamic CO2 adsorption capacity of 11.9
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wt.% (2.7 mmol/g) (corresponding to 8 mL/min, 298 K and 25% v/v CO2). The tested CPM-
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5 showed an outstanding adsorption equilibrium capacity (e.g. 2.3 mmol/g (10.2 wt. %) at
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298 K) compared to other adsorbents, which can be considered as an attractive adsorbent for
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separation of CO2 from flue gas.
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Keywords: Carbon dioxide, CPM-5, Adsorption kinetics, Breakthrough curves, COMSOL Modeling, Parametric study.
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*Corresponding author, Dr. Sohrab Rohani (
[email protected]), Telephone: +1-519-661-4116; Fax: +1-519-661-3498
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1. Introduction
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The release of harmful greenhouse gases such as CO2, CH4, and N2O into the environment is
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a growing concern for the world’s climate change. Carbon dioxide is the paramount contributor
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to the global warming phenomenon. About 60% of global warming attributed to CO2 emissions
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(1). More than 22 billion tons of the annual emission of CO2 gas is resulted from the excessive
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human activities such as fossil fuel combustion, industrial processes and transportations (2).
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Fossil fueled power plants are the largest potential source of CO2 emission. Fossil fuels provide
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81 percent of the world’s commercial energy supply (3). Fossil fuels produce nearly 3x1010
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metric tons (30 petagram) of carbon dioxide annually. About three-quarters of the increase in
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atmospheric carbon dioxide is attributed to burning fossil fuels (4). The current levels of CO2
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concentration in the atmosphere have increased by more than 35% since the industrial revolution,
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i.e. from 280 ppm by volume in pre-industrial times to 368 ppm in 2000 (3), and 388 ppm in
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2010
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atmosphere may contain up to 570 ppm of carbon dioxide in 2100 causing a rise in the mean
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global temperature of around 1.9 °C and an increase in the mean sea level of 3.8 m (5).
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Therefore, effective capture and sequestration of CO2 from post-combustion effluent such as flue
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gas is of immense importance to allow humankind to continue burning fossil fuels for years to
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come until substituting of alternative renewable energies are feasible at competitive and
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reasonable costs (6).
(3). According to the Intergovernmental Panel on Climate Change (IPCC) (4), the
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In this regards, scientists are trying to develop effective systems for CO2 removal from post-
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combustion flue gas by combining the high capacity and selectivity, fast kinetics, mild
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conditions for regeneration, and tolerance to moisture with minimal cost (7). Liquid-phase gas
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absorption using alkanolamine solution (e.g. mono and tri-ethanolamine), is one of the most 2 ACS Paragon Plus Environment
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developed and applied technology for CO2 capturing in industrial scales for decades (6). Some of
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the drawbacks of this process can be expressed as the high energy required for the regeneration
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of the degraded amines under the high temperature treatments, severe corrosion of the
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equipment, and environmental issues related to the alkanolamine volatility (9, 10). Porous
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membrane is another technology that is recently emerged for CO2 capturing. The first
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implementation of membranes technology for gas separation was reported in the 1980s and since
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then membranes have been widely used in many industrial separation processes. Membrane
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based technologies have some disadvantages such as low stability under the reforming
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environment and low separation factor. Since membranes cannot achieve high degree of
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separation, multiple stages are required. Nevertheless, membrane technologies are still under
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development phase (11). In order to overcome the techno economical restrictions of the above-
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mentioned technologies, search for alternative technologies has been prompted. Adsorption in
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porous adsorbents is considered as an alternative viable approach for CO2 capturing. Adsorption
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into solid porous adsorbents is an attractive technology to improve or substitute the current CO2
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absorption technologies due to their high CO2 adsorption capacities, simple and easy to control
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process, low energy consumption, and superior energy efficiency (7, 9, 12). Many adsorbents
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have been developed and studied for CO2 capturing such as zeolites (13), activated carbons (14),
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modified mesoporous silica (15); however the common drawbacks of these conventional
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adsorbents are: high energy consumption for regeneration, low productivity (16) and low CO2
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capacities.
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New porous materials with higher adsorption capacity and selectivity are needed to improve
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the CO2 separation and storage process. Metal organic frameworks (MOFs) have emerged as a
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new class of crystalline porous materials composed of self-assembled metallic species and 3 ACS Paragon Plus Environment
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organic linkers to form three dimensional network structure (16,17). MOFs are renowned
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materials having remarkable high specific surface area, highly divers structural chemistry (16),
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and controlled pore size and shape from microporous (i.e. Angstrom) to mesoporous (i.e.
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nanometers) scale (20). MOFs are under extensive studies for potential industrial applications
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from gas separation, adsorption and storage processes (18), to heterogeneous catalysis (19),
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pharmaceutical manufacturing processes and drug carriers (20).
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Literature review shows that most of the studies on the MOFs as adsorbents for CO2
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adsorption are performed using pure CO2 under high pressure and often at room or sub-ambient
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temperature. It has been reported that the MOF-117, MIL-101 and IRMOF-1 exhibit exceptional
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CO2 storage capacity at high pressure, e.g. 33.5 mmol/g at 40 bar, 40 mmol/g at 50 bar, and 21.7
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mmol/g at 35 bar, respectively (21, 22). However, their capacities are dramatically reduced under
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dynamic conditions and sub-atmospheric pressure. More recently, Zongbi Bao et al. (23) ,
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Sabouni et al.(24) and Saha et al. (25) have reported the CO2 adsorption capacity of 8.61 mmol/g
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at 298 K and low pressure of 1 bar on Mg-MOF-47, 3 mmol/g on CPM-5 and 1.7 mmol/g on
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MOF-177 at room temperature and low pressure of 1 bar, respectively.
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To the best of our knowledge, virtually no research work has been conducted to examine the
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dynamic gas adsorption and kinetic properties of MOFs when exposed to mixture of gases as
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would be the actual case of power plant flue gas streams (26). In addition, the number of studies
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that have examined the experimental operating conditions of the breakthrough data compared
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with mathematical modeling of CO2 capturing onto the MOFs is limited (27-30). Accordingly, it
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is essential to determine the dynamic adsorption capacity of CO2 onto MOFs, which can be
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measured by exposing the adsorbent to a gas stream and detect the arbitrary “breakthrough”
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value corresponding to 10% of the feed concentration. 4 ACS Paragon Plus Environment
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The metal organic framework that is studied in the present work is CPM-5, which is a highly
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porous indium based metal organic framework with a surface area of 2187 m2/g (31). CPM-5
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consists of In3O clusters as metal centres connected by 1, 2, 3-benzenetricaboxylic acid (H3BTC)
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as a linear organic linker. In addition, CPM-5 has three unique cage-within-cage based porous
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structures which contribute to a high CO2 uptake capacity. In our previous work, it has been
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shown that CPM-5 (microwave assisted samples) exhibits a high CO2 adsorption capacity of 2.55
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mmol/g, at 298K and 1 bar with an isosteric heat of adsorption of 36.1 kJ/mol according to the
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isothermal adsorption experimental data (24). Sabouni et al.
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assisted CPM-5 samples exhibited a very high surface area of 2187 m2/g compared to
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solvothermal synthesized samples (e.g. 580 m2/g). In addition, the microwave assisted synthesis
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approach resulted in a dramatic improvement in increasing CO2 adsorption capacity and decreasing the
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synthesis time of CPM-5 metal organic framework from 5 days in conventional solvothermal synthesis to
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ca. 10 min. These advantages have positive impact on the larger scale production for industrial
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applications such as lower energy consumption and very short synthesis time.
(24) showed that microwave
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The CPM-5 shows a stable structure after exposure to ambient conditions for several weeks. It was
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found the XRD pattern remains unchanged for the aged samples for several weeks (tested for up to 4
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weeks) under laboratory conditions (296 K and relative humidity of 62%) (24). As a result, CPM-5 can
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be considered as a practical attractive MOF candidate with long shelf life for industrial
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application.
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To the best of our knowledge, no literature has reported the dynamic adsorption capacity of
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CO2 onto CPM-5 from experimental breakthrough data coupled with the mathematical modeling.
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The main objective of the present work is to measure the dynamic adsorption of CO2 onto
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CPM-5 and compare the experimental results with the mathematical modeling of the 5 ACS Paragon Plus Environment
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breakthrough curve using COMSOL program. Furthermore, the effect of several experimental
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operating conditions on the breakthrough value is studied. The experimental breakthrough curves
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are obtained under different operational conditions including: a) the various flow rates of the
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gaseous mixture of CO2 and N2 (i.e. 8 mL/min and 32 mL/min), b) feed concentration of CO2
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(i.e. 12.5 % and 25 % by volume), and c) adsorption temperature (i.e., 298 K and 318 K).
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2. Experimental
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2.1. Synthesis of CPM-5
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The CPM-5 was synthesized using microwave irradiation as heating source (Discovery
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system model of CEM Laboratory Microwave, USA) and activated according to the procedure
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reported previously (231). A mixture of In(NO3)3.xH2O (0.2 g), and 1, 2, 3-benzenetricaboxylate
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(0.17 g) was dissolved in H2O/DMF (1/4.2 v/v) solvent. After mixing and dissolving the
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reactants using a magnetic stirrer, the clear solution was transferred into a 40 mL glass reactor
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and microwave irradiated at power of 300 W for 10 min at 423 K. After cooling to room
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temperature the produced crystallites were separated by means of centrifugation at 20,000 rpm
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for 20 min. CPM-5 washed with fresh 1:1 solution of H2O/DMF three times by repeated
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dispersing and centrifuging to remove all of the un-reacted chemicals. Subsequently, the CPM-5
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powders were dried at 373 K for 1 h in an electrical oven and then characterized.
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2.2. Characterization of CPM-5
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The CPM-5 crystalline structure was confirmed by powder x-ray diffraction (XRPD)
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analysis using a Rigaku – MiniFlex powder diffractometer (Japan; CuKα = 1.54059 Å) over 2θ
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range of 5° to 40° with step width of 0.02°. Adsorption equilibrium of CO2 on CPM-5 was
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measured volumetrically using a BET instrument (Micromeritics ASAP 2010, USA) at CO2 6 ACS Paragon Plus Environment
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pressure up to 1 bar and two temperatures of 298, and 318 K. Ultra high pure CO2 and N2
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(Praxair Canada Inc.) were used for the adsorption measurements and backfill gas, respectively.
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Further characteristic data of the as-synthesized CPM-5 including the crystallites morphology,
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shape and size as well as its FTIR spectrum were reported in the author’s previous work (24).
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2.3. CO2 adsorption apparatus and Breakthrough measurements
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A simplified scheme of the system (BTRS-Jr-PC; Autoclave engineers, Division of Sap-
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tite, Inc., Erie, PA, USA) used to conduct the breakthrough experiments is shown in Figure S1,
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Supportive Information. The experimental setup consisted of: 1) a stainless steel fixed bed
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adsorption column (ID= 0.792 m, L= 0.153 m) equipped with inlet and outlet filters and a full-
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length internal thermo-well; 2) a bypass line for measuring the feed concentration; 3) flow-
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meters/controllers to control the flow-rates of the inlet gases; 4) a gas mixture; the inlet gases
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mix thoroughly before flowing through the adsorbent bed, 5) and an on-line gas chromatograph
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(CP-3800 ,Varian Inc., Lake Forest, CA) equipped with a TCD detector, and a CP7429 capillary
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column to measure the CO2 concentration. CO2 (99.995%) and N2 (99.995 %) were used as feed
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gas mixture (Praxair Canada Inc., Sarnia, ON, Canada). The experiments were conducted as
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follows: 100 mg of CPM-5 was loaded into the reactor and fixed between glass wool plugs. Prior
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to the adsorption tests, the CPM-5 adsorbent was activated under nitrogen (N2 flow rate =100
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mL/min) at 298 K until no other gases were detected at the reactor outlet. The N2 flow rate was
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set to the desired value according to the experimental conditions (e.g. 7 mL/min in the case of
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12.5 v% CO2). The CO2 was introduced at a flow rate of 1 mL/min resulting in a gas mixture
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with CO2 content of 12.5 % by volume. The other CO2 feed concentration (i.e. 25% mixture) was
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obtained in the same manner. The breakthrough value was defined to be at 10% of the CO2 feed
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concentration. In order to fully regenerate the saturated adsorbent for the next experiment, the 7 ACS Paragon Plus Environment
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column temperature was raised to 373 K and N2 flow rate of 100 mL/min was purged for 1 h.
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The experimental conditions for the breakthrough curve are summarized in Table 1. Table 1,
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3. Breakthrough curve model
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The fixed bed reactor packed with porous spherical particles of CPM-5 was subjected to an
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inert gas flow under steady state. To formulate a breakthrough curve for this system, the
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following mathematical model was developed based on the following assumptions: 1) the flow
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pattern is described by the axially dispersed plug flow model. 2) The system operates under
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isothermal conditions. 3) The frictional pressure drop through the column is negligible. 4) The
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adsorption equilibrium isotherm is described by Freundlich isotherm. 5) The adsorbent particles
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are spherical and homogenous in size and density. 6) The velocity of the gas is constant. The
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driving force is the concentration gradient of the adsorbed phase and the diffusion coefficient is
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constant. Under the above assumptions, a set of governing scaled equations and appropriate
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scaled initial and boundary conditions can be established as follows:
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Mobile gas phase:
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The mass balance of the adsorbed gas on the stationary crystal phase (CPM-5):
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where:
!"
0
Eq. (1)
Sh y ∗ y
Eq. (2)
# %& '
1 *+ ( , *+ 8 ACS Paragon Plus Environment
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-.
%/ ' 0
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Thermodynamic equilibrium:
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1+∗
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Scaling:
1+
3/5 3/5 67
2
8
Eq. (3)
9+ =+ ? AB ; ; @ : =/ %/ %/
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The initial and boundary conditions:
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t=0
0
