Article pubs.acs.org/est
Adsorption Separation of Carbon Dioxide from Flue Gas by a Molecularly Imprinted Adsorbent Yi Zhao,*,† Yanmei Shen,‡ Guoyi Ma,§ and Rongjie Hao† †
School of Environmental Science & Engineering, North China Electric Power University, Baoding 071003, People’s Republic of China ‡ Shenhua Guohua (Beijing) Electric Power Research Institute Co., Ltd., Beijing 100025, People’s Republic of China § State Grid Jibei Electric Power Co., Ltd. Maintenance Branch, Beijing 100053, People’s Republic of China S Supporting Information *
ABSTRACT: CO2 separation by molecularly imprinted adsorbent from coal-fired flue gas after desulfurization system has been studied. The adsorbent was synthesized by molecular imprinted technique, using ethanedioic acid, acrylamide, and ethylene glycol dimethacrylate as the template, functional monomer, and cross-linker, respectively. According to the conditions of coal-fired flue gas, the influencing factors, including adsorption temperature, desorption temperature, gas flow rate, and concentrations of CO2, H2O, O2, SO2, and NO, were studied by fixed bed breakthrough experiments. The experimental conditions were optimized to gain the best adsorption performance and reduce unnecessary energy consumption in future practical use. The optimized adsorption temperature, desorption temperature, concentrations of CO2, and gas flow rate are 60 °C, 80 °C, 13%, and 170 mL/min, respectively, which correspond to conditions of practical flue gases to the most extent. The CO2 adsorption performance was nearly unaffected by H2O, O2, and NO in the flue gas, and was promoted by SO2 within the emission limit stipulated in the Chinese emission standards of air pollutants for a thermal power plant. The maximum CO2 adsorption capacity, 0.57 mmol/g, was obtained under the optimized experimental conditions, and the SO2 concentration was 150 mg/m3. The influence mechanisms of H2O, O2, SO2, and NO on CO2 adsorption capacity were investigated by infrared spectroscopic analysis. 85%.11 Additionally, corrosion and solvent degradation are also important disadvantages relevant to MEA absorption.12,13 As an alternative process, adsorption by solid sorbents has been claimed to be feasible for CO2 capture at an industrial scale, and it has the potential to reduce the power consumption of CO2 capture and avoids the shortcomings of absorption.14−16 A wide range of adsorbents have been developed by modifying the surface chemistry of carbons and mesoporous silica with amine materials.17−20 However, most of these adsorbents are still hard to be applied in practical CO2 capture processes, because they cannot meet the following requirements simultaneously:21,22 (1) high selectivity and adsorption capacity for CO2; (2) low energy penalties during regeneration; (3) stable adsorption capacity of CO2 after repeated adsorption/desorption cycles; and (4) the CO2 adsorption capacity cannot be inhibited at the presence of gas impurities like SO2, NOx, O2, and H2O. A kind of molecularly imprinted CO2 adsorbent (MIP) has been developed in our lab, using ethanedioic acid, acrylamide, and ethylene glycol dimethacrylate as template, functional monomer, and cross-linker, respectively, by molecular imprinting technology.23−25 Molecular imprinting is a method of
1. INTRODUCTION Carbon dioxide (CO2) emission due to fossil fuel combustion is increasing day by day, causing serious air pollution and greenhouse effects and threatening the global environment. Power generation, which is the largest CO2 stationary emission source, emits 23 Gt (CO2) /year, approximately 26% of the total emissions.1,2 In China, the installed capacity of coal-fired power generation unit was around 7.1 hundred million kW by the end of 2010, accounted for 70% of the total installed capacity.3 Most of the coal-fired power plants are newly constructed, after 2000, and installed with a flue gas dust removal device and desulfurization device. On the basis of this situation in China, postcombustion CO2 capture technology is one of the most effective methods to meet the CO2 reduction demand in the short term because of its maturity and simplicity and lack of need of the innovation of an existing boiler.4−6 In postcombustion processes, CO2 is traditionally separated from flue gas by chemical absorption, such as monoethanolamine (MEA) absorption.7,8 The industrial feasibility of the process has been demonstrated in pilot plants. The advantage of it is the production of a relatively pure CO2 stream, but it also has several shortcomings. In particular, the energy consumption of the process is very high, i.e., 3.7 GJ/t of CO2,9 reducing the electricity output of a coal-fired power plant by 23%.10 Also, the cost of it is horrendous. For a pulverized coal power plant, the increase in cost of electricity production by adopting a CO2 capture process is estimated to be 40− © 2014 American Chemical Society
Received: Revised: Accepted: Published: 1601
September 9, 2013 December 12, 2013 January 10, 2014 January 10, 2014 dx.doi.org/10.1021/es403871w | Environ. Sci. Technol. 2014, 48, 1601−1608
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inducing molecular recognition properties in synthetic polymers. The generated molecular imprinting polymers normally have excellent adsorption selectivity to the target molecule, which is a necessary property for CO2 capture. The characteristics of the MIP were detected by N2 adsorption experiment, thermo-gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The surface area, pore volume, and pore size obtained by N2 adsorption experiment are 215.37 m2/g, 0.655 mL/g, and 12 nm, respectively. TGA tests showed that MIP displayed good thermo-stability at 200 °C. FT-IR spectra proved that a large number of amine groups distribute on the surface of MIP. The MIP exhibited high CO2 capacity and high CO2/N2 selectivity. CO2 adsorption on MIP is a physical reaction. However, different from the experimental gas we have used before, the flue gas emitted from power plants is of complicated operation conditions and gas composition. Thus, before MIP was used in a scalable CO2 capture process in a power plant, a batch of CO2 simulation experiments should be carried out by fixed bed equipment to see whether MIP is suitable under practical flue gas conditions. In this work, the potential factors influencing CO2 capacity, including adsorption temperature; desorption temperature; gas flow rate; and concentrations of CO2, H2O, O2, SO2, and NO in coal-fired flue gas were studied by fixed bed breakthrough experiments. The results showed that the CO 2 adsorption capacity increases with CO 2 concentration increase and temperature decrease, and varies slightly with gas flow rate. It was nearly unaffected by the presence of H2O, O2, and NO in flue gas, and promoted by SO2 within the emission limit stipulated in China.
Figure 1. Diagram of experimental apparatus for fixed-bed adsorption: (1) CO2 cylinder; (2) N2 cylinder; (3) SO2 cylinder or others; (4, 8) flow meter; (5) fixed-bed adsorber; (6) CO2 analyzer; (7) steam generator; and (9) heating box; V1−V3 pressure reducing valves; V3− V10 stop valves; T: thermal couples.
shown in Table 1. The temperature of coal-fired flue gas after desulfuration is around 60 °C, the concentration of CO2 is 10− 15%, and the rest composition is mostly N2 by volume. Thus, in a basic experiment, the composition of simulated flue gas was determined to be 13% CO2/87% /N2, the adsorption temperature and the gas flow rate was controlled at 60 °C and 170 mL/min. Additionally, the desorption run was conducted at 120 °C in N2 flow to ensure complete regeneration of MIP. A 3.0 g portion of prepared adsorbent was loaded into a quartz adsorber (10 mm O.D.; 5 mm I.D.) wound with heating tape. Three thermal couples located at inlet and outlet as well as inside of the cell were used to monitor the temperatures during operation. The temperature could be controlled within ±0.5 °C. Before CO2 adsorption, the valves at both ends of the adsorber column were stopped until the temperature was raised up to 120 °C, and then opened to perform desorption for 4 h in N2 atmosphere with a flow rate of 100 mL/min. During adsorption, the gas flow rate of the simulated flue gas was controlled by mass flow meter (Rotamass, RCCS, German). The CO2 concentration in effluent gas stream at the outlet of the absorber column was measured by a CO2 infrared analyzer (Zhonghui, NKJH-3860B, China). The CO2 breakthrough curves were made according to the concentration of CO2 outlet the fixed-bed varying with time. The CO2 adsorption capacity was calculated by integrating the CO2 breakthrough curves. Experiment under every one operation condition was carried out for 5 times, the final breakthrough curve corresponding to this condition and the CO2 adsorption capacity were the average value of the 5 times. 2.3.2. Contrast Experiments. In the 8 sets of contrast experiments shown in Table 1, experimental conditions: adsorption temperature, desorption temperature, gas flow rate, and the concentration of CO2, H2O, O2, SO2, and NOx, were varied in succession. By contrasting the breakthrough curves of 8 sets of experiments with the basic experiment, the best operating conditions, including adsorption temperature; desorption temperature; gas flow rate; and CO2 concentration were determined; and the effects of H2O, O2, SO2, and NOx on CO2 adsorption were investigated. It is important to note that the concentrations of H2O in Set 5 were controlled by the temperature of steam generator and measured by hygrometer. The range of O2 concentration added
2. EXPERIMENTAL SECTION 2.1. Chemicals. Ethylene glycol dimethacrylate (EGDMA) was obtained from Aladdin-Reagent (Shanghai, China); ethanedioic acid, acrylamide (AAM), azodiisobutyronitrile (AIBN), acetonitrile (AN), toluene, methanol, and hydrochloric acid (HCl) were purchased from Kermel Chemical Reagent Ltd. (Tianjin, China) and were analytical reagent grade. High purity water (>18 MΩ) was produced by lab water purification system (Changfeng Co., Ltd., Beijing). All gases used in this work had purity higher than 99.99%, and were supplied by Beiyang Co., Ltd. 2.2. Synthesis of MIP Adsorbent. The MIP adsorbent was synthesized by the following method: 3 mmol ethanedioic acid and 12 mmol AAM were dissolved in 10 mL AN/toluene solution for 2 h with agitation followed by adding 20 mmol of EGDMA and 0.3 mmol AIBN. The mixture was then degassed by ultrasonic device for 15 min and purged with N2 for another 10 min to remove oxygen. After that, the mixture was sealed and reacted for 24 h at 60 °C. The resultant polymers were ground and screened to 50−150 μm. The particles were washed with HCl/methanol (1:9, v/v) solution to remove ethanedioic acid and then filtered off. The washing procedure was repeated several times until the template could not be detected in the filtrate. The remaining polymer particles were washed with high purity water to neutral and then dried overnight under vacuum at 60 °C. 2.3. Simulation Experiments. 2.3.1. Introduction of Basic Experiment. A series of simulation experiments were carried out on the fixed bed equipment shown in Figure 1. According to the properties of flue gas after wet type desulphurization system of coal fired power plant, the basic experiment operation conditions of fixed bed were set and 1602
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Table 1. Operation Conditions of Fixed Bed Experiments experiments
adsorption temp. (°C)
desorption temp. (°C)
conc. of CO2
flow rate (mL/min)
basic set 1
120 120
13% 13%
170 170
0 0
0 0
0 0
0 0
set 2
60 45 60 75 90 60
13%
170
0
0
0
0
set 3
60
60 80 100 120 80
170
0
0
0
0
set 4
60
80
17% 13% 9% 5% 13%
0
0
0
0
set 5
60
80
13%
210 170 130 170
0
0
0
set 6
60
80
13%
170
rh rh rh rh rh 0
0
0
set 7
60
80
13
170
0
8 9 10 11 12 0
0
set 8
60
80
13%
170
0
0
50 100 150 200 2000 0
H2O (%rh)
20% 40% 60% 80% 100%
O2 (mL/min)
SO2 mg/m3
NOx mg/m3
50 100 150 200 2000
The surface chemistry of adsorbents before and after the simulation experiments was analyzed from Fourier transform infrared (FT-IR) spectra, which were obtained using FT-IR spectrometer (Thermo Nicolet 380, U.S.).
in the simulated gas of Set 6 was determined according to the practical concentration of O2 in flue gas. The concentrations of SO2 and NOx added in flue gas of Set 7 and Set 8 was determined according to the Chinese emission standard of air pollutants for thermal power plants implemented since 2012. The emission limit of SO2 was 200 mg/m3 for the existing coalfired boiler and 100 mg/m3 for new established coal-fired boiler, the emission limit of NOx was 100 mg/m3. Thus, the experimental concentrations of SO2 in Set 7 were 0, 50, 100, 150, and 200 mg/m3 respectively, which were in the range from 0 to 200 mg/m3. Additionally, to further reveal the influence mechanism of SO2 on CO2 adsorption, the CO2 adsorption capacity of MIP under high SO2 concentration (2000 mg/m3) was also investigated. The experimental concentrations of NOx in Set 8 were 0, 25, 50, 75, and 100 mg/m3 respectively, which were in the range of 0 to 100 mg/m3. The effect of NOx at high concentration (2000 mg/m3) on the CO2 adsorption property was also investigated to further reveal the influence mechanism. As the NO content accounts for over 90% of NOx in coal-fired flue gas, pure NO was used to represent NOx in the experiments to investigate the effect of NO x on CO 2 adsorption.
3. RESULTS AND DISCUSSION 3.1. Results of Basic Experiment. The breakthrough curve obtained by basic experiment is presented in Figure 2. Three parameters tb, qb, and Za, are used to describe the breakthrough curve, in which, tb is defined as the time corresponding to Coutlet/Cinlet = 0.1, qb is the overall CO2 uptake calculated by integration of the breakthrough curve, and Za is the length of the mass transfer zone calculated from the formula raised by Treybal: 26 ⎤ ⎡ te − tb Za = Z ⎢ ⎥ ⎣ te − 0.5(te − tb) ⎦
(1)
where, te is the time when Coutlet/Cinlet is equal to 0.9, Z is the length of packed bed, 52 mm. It is important to note that the larger the value of tb and qb, the higher the CO2 capacity becomes; the larger the value of Za, the lower the effective utilization rate of the adsorbent becomes in industral operation. 1603
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Table 2. CO2 Adsorption Capacity of MIP under Different Operating Conditions experiments basic set 1
varying parameters adsorption temp. (°C)
set 2
desorption temp. (°C)
set 3
CCO2
set 4
flow rate (mL/min)
Figure 2. The breakthrough curve obtained by basic experiment.
values
qb (mmol/g)
tb (s)
Za (mm)
45 60 75 90 60 80 100 120 17% 13% 9% 5% 210 170 130
0.49 0.55 0.49 0.33 0.19 0.41 0.49 0.49 0.49 0.52 0.49 0.44 0.38 0.48 0.49 0.49
81 97 81 60 37 59 81 81 81 52 81 114 150 46 81 110
14 17 14 12 12 19 14 14 14 31 14 7 5 33 14 7
Figure 3. The breakthrough curves obtained under different operation conditions. 1604
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Figure 6. CO2 breakthrough curves under different O2 flow rate.
Figure 4. CO2 breakthrough curves at the relative humidity (rh) of 0%, 20%, 40%, 60%, 80%, and 100% rh.
Figure 7. CO2 breakthrough curves at different SO2 concentration: 0, 50, 100, 150, 200, and 2000 mg/m3.
be 60 °C, because it can balance the adsorption capacity and effective utilization of adsorbent, and most of all, it avoids the energy consumption resulting from flue gas heating or cooling. From the results of experiments in Set 2, no obvious difference was appeared on the breakthrough curves of CO2 as long as the desorption temperature of MIP is above 80 °C, suggesting that 80 °C is enough to ensure complete regeneration of MIP. However, when desorption temperature was reduced to 60 °C, complete regeneration of MIP cannot be realized, because qb for MIP decreased from 0.49 mmol/g to 0.41 mmol/g. From the above, 80 °C should be the optimal desorption temperature. From the results of experiments in Set 3, qb gains the highest value under the highest feed CO2 concentration, and decreases with the decrease of CO2 concentration. This tendency is consistent with Henry’s law. Additionally, Za also presented a decreasing tendency with the decrease of CO2 concentration, indicating high CO2 concentration is not beneficial to the adsorbent effective utilization. This phenomenon can be explained by the diffusion mechanism of CO2 in a fixed-
Figure 5. FTIR spectrum of fresh MIP and MIP treated by flue gases containing H2O, O2, SO2, and NOx, respectively (MIP/H2O, MIP/O2, MIP/SO2, and MIP/NOx).
The calculated values of tb, qb, and Za in basic experiment are 81s, 0.49 mmol/g, and 14 mm, respectively. 3.2. Determination of the Best Operating Conditions. The breakthrough curves obtained by the first 4 sets of experiments are shown in Figure 3. The corresponding values of tb, qb, and Za are shown in Table 2. From the results of experiments in Set 1, tb and qb decreased greatly with the increase of temperature, which is the consequence of exothermic reaction. This is further evidence that adsorption of CO2 is a weak physical-sorption interaction, which is affected significantly by temperature. Za also presented an opposite trend with the increase of temperature, indicating that lowing temperature could promote the efficient use of the adsorbent. Thus, an overall consideration was given here, as the flue gas temperature after wet desulphurization is close to 60 °C, an optimal adsorption temperature in industrial operation should 1605
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8, were carried out to investigate the effect of the gas impurities on CO2 adsorption performance. 3.3.1. Effect of H2O on CO2 Adsorption of MIP. Figure 4 shows the CO2 breakthrough curves of the simulated flue gas under the relative humidity of 0%, 20%, 40%, 60%, 80%, and 100%. The calculated results of CO2 adsorption capacity of MIP under five humidity conditions were 0.48, 0.49, 0.48, 0.48, and 0.47 mmol/g, respectively. With the increase of humidity in flue gas, no obvious variation of CO2 adsorption capacity was observed. This result could be explained from the surface property of MIP and the CO2 adsorption mechanism. As we have confirmed in our previous paper,23 a large number of amine active sites exist on the surface of MIP, and they play an important role on CO2 adsorption. The amide groups can interact with water molecules in theory. Additionally, this interaction was interfered and reduced by a large number of hydrophobic groups around the amides. However, the hydrogen bond between water molecules will weaken the interaction between the amide groups and water molecules, making the absorbed water on the surface of amide groups much smaller, even negligible. Therefore, the water molecules do not occupy the effective amide groups for adsorbing CO2, and the CO2 capacity of MIP cannot be influenced by H2O. However, the CO2 molecules are nonpolar, the interaction between CO2 and amide groups was nearly not affected by hydrophobic groups. The surface wettability performance of MIP was determined by the ratio of hydrophilic functional group NH2CO− to the hydrophobic functional group −(CH 2 −CH) n −. If the NH2CO− functional group dominated the surface structure, then the MIP would show strong hydrophilic ability, the H2O molecule in flue gas would be attracted on the surface and exert an inducing effect on the NH2CO− group due to strong polarity, resulting in the variations of dipole moment and infrared spectrum of NH2CO− and forming new chemical species. Figure 5 shows the FT-IR spectrum of MIP after CO2 adsorption experiments carried out at 100% relative humidity of simulated flue gas, which is denoted as MIP/H2O. Comparing with the fresh adsorbent MIP, the FI-TR spectrum of MIP/ H2O showed no obvious change except that the absorption peak at 3450 cm−1 shifted to 3440 cm−1, which could be due to the overlap of the N−H deformation vibration of the amide group and O−H stretching vibrations of water molecules in the pore channels of MIP. This result suggests that no chemical reaction has occurred on the surface of the adsorbent under moisture conditions. According to the characteristic peak of amide group at 1730 cm−1, the dipole moment of the amide group was not changed by water molecules, which further
Figure 8. CO2 breakthrough curves at different NO concentration: 0, 50, 100, 150, 200, and 2000 mg/m3.
bed.27 As for the high CO2 concentration airflow, the hydrodynamic behavior of the airflow was changed, and the diffusion resistance becomes higher because of the increase of gas viscosity, leading to the decrease of diffusion rate and then the decrease of the mass transfer zone. In general, the middle two concentrations, 13% and 9% could balance the adsorption capacity and effective utilization of adsorbent. As the CO2 accounts for 10−15% of the total amount of industrial coalfired flue gas, 13% should be the best choice for CO2 concentration, because it is much more consistent with the actual situation. From the results of experiments in Set 4, it is clear that qb varies slightly with the decrease of gas flow rate, but Za drops obviously which may be due to the decrease of gas−solid contact time. It can be seen that high gas flow rate cannot change the adsorption capacity of MIP, but lowers the effective utilization. Considering that the adsorption column would be an enlarged device if the gas flow rate is too slow in industry, the medium rate, 170 mL/min should be the optimal choice in experiments. 3.3. Effect of Flue Gas Impurities on CO2 Adsorption Performance of MIP. Under the optimal operation conditions, we have determined (adsorption at 60 °C, desorption at 80 °C, CO2 concentration is 13%, gas flow rate is 170 mL/min), the following sets of experiments, Set 5 to Set
Table 3. Effects of H2O, O2, SO2, and NOx on the CO2 Adsorption Capacity of MIP sets
gas
set 5
H2O
set 6
O2
addition 20% 40% 60% 80% 100%
rh rh rh rh rh 8 9 10 11 12
mL/min mL/min mL/min mL/min mL/min
CO2 capacity (mmol/g)
sets
gas
0.48 0.49 0.48 0.48 0.47 0.49 0.48 0.48 0.48 0.49
set 7
SO2
set 8
NOx
1606
addition 50 100 150 200 2000 50 100 150 200 2000
mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3 mg/m3
CO2 capacity (mmol/g) 0.51 0.54 0.57 0.55 0.43 0.48 0.48 0.49 0.49 0.50
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peak at 1730 cm−1 is formed by the overlap of N−H deformation vibration of amine group with CO stretching vibration. The formation of the shoulder peak implies the dipole moment change and red shift of N−H, which is a consequence of strong polarization and induction effect of SO2 molecule around. The generated amine groups gain an improved proton trapping ability, making it more effective to capture CO2. However, after the maximum value at 150 mg·m−3 SO2 concentration, the CO2 capacity decreased significantly with the increase of SO2 concentration. It can be explained that SO2 molecules competed with CO2 to bind with active adsorption sites in MIP adsorbent. 3.3.4. Effect of NO on CO2 Adsorption of MIP. Figure 8 shows the experimental breakthrough curves of CO2 affected by NO. The NO concentration added in the stimulated flue gas was 50, 100, 150, 200, and 2000 mg/m3, respectively. Compared to the result of blank experiment, the breakthrough curves skewed slightly to the right after adding NO. The corresponding CO2 capacity (shown in Table 3) presents a slight growth, the maximum value of which is 0.50 mmol/g occurring at 2000 mg/m3. The increase of adsorption capacity indicates the property change of surface groups. As shown in Figure 5, comparing with the fresh MIP, the FT-IR spectra of MIP treated by the simulated flue gas with 2000 mg·m−3 NO (denoted as MIP/ NO), splits a new shoulder peak at 1730 cm−1, confirming the dipole moment change of N−H. Similar to SO2, the dipole moment change of N−H can be explained from the polarity property of NO. Within 2000 mg/m3, the NO molecules adhered on the surface of adsorbent exerts a polarization and induction effect due to its polarity, resulting in the changes of dipole moment of the around surface amine groups. The new generated amine groups gain an improved proton trapping ability, making it more effective to capture CO2. However, the incentive effect of NO on CO2 capture is not as obvious as that of SO2, which may be because the polarity of NO (0.53 × 10−30 C·m) is much lower than that of SO2 (5.33 × 10−30 C·m). Another difference with SO2 is that no distinct declining trend appears with the increase of NO concentration within 2000 mg/m3. This discrepancy can be related to the acid−base property of the two gases. NO is not a kind of acidic oxide, and it has a low ability to compete with CO2 to bind with active adsorption sites of the MIP adsorbent.
proved that the water molecules did not react with the surface amide groups. These phenomena could be explained by the existence of huge −(CH2−CH)n− groups generating the hydrophobic property of the adsorbent surface, preventing the interaction between water molecules and amide groups. 3.3.2. Effect of O2 on CO2 Adsorption of MIP. Figure 6 reports six CO2 breakthrough curves obtained at different O2 flow rates, namely 0, 8, 9, 10, 11, and 12 mL/min, respectively. Only slight changes were observed on these curves with the variation of O2 concentration, which can be attributed to experimental error. This result reveals that the O2 molecules in the simulated flue gas exert no impact on CO2 adsorption. The corresponding CO2 capacities calculated from the breakthrough curves were 0.49, 0.48, 0.48, 0.49, and 0.49 mmol/g respectively, which is consistent with the trend of breakthrough curves, further confirming that O2 does not influence the CO2 capacity. The phenomenon can be explained from the aspect of the property of O2 molecule. The diatomic O2 molecule is nonpolar, chemically and physically stable at normal temperature, making chemical adsorption between O2 and adsorbent difficult. Figure 5 shows the FTIR spectra of MIP treated by simulated flue gas with O2 flow rate of 12 mL/min (MIP/O2 for short). Compared with the FT-IR spectra of fresh MIP adsorbent, the spectra of MIP/O2 shows no obvious change, indicating that the dipole moment of the surface functional groups was not influenced by O2, confirming that no chemical reaction occurred between O2 and adsorbent surface. Additionally, researches have showed that O2, N2, and Ar have almost the same adhesion strength on most adsorbents, except for zeolite, because their polarizabilities are almost the same, namely 1.74 × 10−24, 1.58 × 10−24, and 1.63 × 10−24 cm3, respectively. The adsorption capacity of O2 of MIP is nearly identical to that of N2, which can be ignored, being comparable to CO2. 3.3.3. Effect of SO2 on CO2 Adsorption of MIP. Figure 7 also presents six experimental breakthrough curves of CO2, which were obtained by experiments operated under the same conditions as the basic experiment, except that SO2 was added in the simulated flue gas. The corresponding SO2 concentrations to the six curves are 0, 50, 100, 150, 200, and 2000 mg·m−3, and the calculated CO2 capacities of MIP are 0.51, 0.54, 0.57, 0.55, and 0.43 mmol·g−1, respectively. As depicted, it changes drastically over the SO2 concentration. The CO2 capacity increases at first with the increase of SO2 content in gas flow, the maximum value appears at 150 mg·m−3, and decreases rapidly afterward. Within 200 mg/m3, which is the SO2 emission limit stipulated in the Chinese emission standard of air pollutants for existing thermal power plants, the CO2 adsorption capacity of MIP is promoted by SO2. This positive role of SO2 can be explained in terms of its physical property. At the initial phase of SO2 adsorption, a tiny amount of SO2 molecules are adhered onto the surface of the adsorbent and exert a strong polarization and induction effect due to its strong polarity, resulting in changes of dipole moment of surface groups around which one can refer to the FT-IR spectra. Figure 5 presents the FT-IR spectra of fresh MIP and MIP treated by simulated flue gas with 100 mg·m−3 SO2 (MIP/SO2 for short). By comparison, it can be found that no sulfur containing functional groups were formed on the surface of MIP, which proves that SO2 does not react with the surface amide groups at low concentrations. A new shoulder peak splits from the strong peak at 1730 cm−1 in the spectra of MIP/SO2. The original
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ASSOCIATED CONTENT
S Supporting Information *
. Adsorption capacity of MIP in 20 cycles of CO2 adsorption/ regeneration (Figure S1); comparison with other adsorbents; details of the FTIR experiments; regeneration experiments; and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-312-7522343; fax: +86-312-7522192; e-mail:
[email protected]. Notes
The authors declare no competing financial interest. 1607
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(21) Mondal, M. K.; Balsora, H. K.; Varshney, P. Progress and trends in CO2 capture/separation technologies: A review. Energy 2012, 46, 431−441. (22) Zhang, J.; Xiao, P.; Li, G.; Webley, P. A. Effect of flue gas impurities on the performance of a chemical looping based air separation process for oxy-fuel combustion. Energy Procedia 2009, 1, 1115−1122. (23) Zhao, Y.; Shen, Y. M. Synthesis and CO2 adsorption properties of molecularly imprinted adsorbents. Environ. Sci. Technol. 2012, 46 (3), 1789−1795. (24) Zhao, Y.; Shen, Y. M. Effect of chemical modification on carbon dioxide adsorption property of mesoporous silica. J. Colloid Interface Sci. 2012, 379 (1), 94−100. (25) Zhao, Y.; Shen, Y. M.; Bai, L. Carbon dioxide adsorption on polyacrylamide-impregnated silica gel and breakthrough modeling. Appl. Surf. Sci. 2012, 261, 708−716. (26) Treybal, R. E. Mass-Transfer Operations; McGraw-Hill Book Company: New York, 1955. (27) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984.
ACKNOWLEDGMENTS The authors appreciate the financial support of this research by the Major State Basic Research Development Program of China (973 Program, No. 2006CB200300-G).
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