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CO2 bubble absorption enhancement in methanol-based nanofluids
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
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w w w . i i fi i r . o r g
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CO2 bubble absorption enhancement in methanol-based nanofluids Jae Won Lee a, Jung-Yeul Jung b, Soon-Geul Lee a, Yong Tae Kang a,* a
Department of Mechanical Engineering, Kyung Hee University, Yong In, Gyeong-gi 446-701, Republic of Korea Marine Safety & Pollution Response Research Department, Maritime & Ocean Engineering Research Institute, KORDI, Daejon 305-343, Republic of Korea b
article info
abstract
Article history:
In this study, the nanoparticles (i.e. SiO2 and Al2O3 nanoparticles) and methanol are
Received 21 April 2011
combined into SiO2/methanol and Al2O3/methanol nanofluids to enhance the CO2
Received in revised form
absorption rate of the base fluid (methanol). The absorption experiments are performed in
16 July 2011
the bubble type absorber system equipped with mass flow controller (MFC), mass flow
Accepted 9 August 2011
meter (MFM) and silica gel (which can remove the methanol vapor existing in the outlet
Available online 16 August 2011
gases). The parametric analysis on the effects of the particle species and concentrations on CO2 bubble absorption rate is carried out. The particle concentration ranges from 0.005 to
Keywords:
0.5 vol%. It is found that the CO2 absorption rate is enhanced up to 4.5% at 0.01 vol% of
Absorption
Al2O3/methanol nanofluids at 20 C, and 5.6% at 0.01 vol% of SiO2/methanol nanofluids at
Enhancement
20 C, respectively. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Bubble Carbon dioxide Methanol Particle
Ame´lioration de l’absorption des bulles de CO2 dans les nanofluides a` base de me´thanol Mots cle´s : Absorption ; Ame´lioration ; Bulle ; Dioxyde de carbone ; Me´thanol ; Particule
1.
Introduction
The shortage of oil will increase the prices of the crude oil, petrochemicals and natural gas in near future. So the synthetic natural gas (SNG) from the coal is one of the promising alternative energy resources. An integrated gasification combined cycle (IGCC) is a power plant system using
the synthesis gas (syngas). In the IGCC, it is required to remove the acid gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S) from valuable feed gas streams. By doing so, the feed gas is made more suitable for combustion and further processing. The ways to remove the CO2 are absorption, adsorption, membranes, cryogenics, chemical looping, etc. (Figueroa
* Corresponding author. Tel.: þ82 31 201 2990; fax: þ82 31 202 3260. E-mail address: [email protected] (Y.T. Kang). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.08.002
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
Nomenclature MFC MFM _ m R t
mass flow controller mass flow meter mass flow rate, kg s1 absorption ratio absorption time, s
et al., 2008; Kim et al., 2008; Chang et al., 2009; Ahmad et al., 2010; Dennis and Scott, 2010; Zhang et al., 2010) So far, the methods of membranes, cryogenics, chemical looping, etc., have not been developed to a practical stage. Currently, the most used ways to remove CO2 are absorption and adsorption methods. However, the adsorption method needs too much energy for desorption process and is not proper for large-scale systems. The absorption methods are divided into physical and chemical types according to the types of absorbents. The chemical absorption type is suitable for atmospheric pressure. Therefore, this type of absorption is suitable to remove CO2 in the exhaust gas. Nonetheless, there are problems such as degradation of the absorbent and the need of much energy for regeneration. However, the physical absorption method is used in the IGCC system due to its suitability for high pressure. Recently, Rectisol system, that is physical absorption type, is used prevalently to remove the acid gases in the SNG production system. This system uses methanol as an absorbent. The advantages of methanol absorbent are cheap and selective absorption of acid gases such as CO2, H2S and COS (Korens et al., 2002; Ranke and Mohr, 1985). In the Rectisol system, the temperature of methanol absorbent should be maintained at least about 20 C to increase the absorption rate according to the Henry’s solubility law. Therefore, huge amount of energy is required to keep such a low temperature of methanol in the Rectisol system.
Subscripts abs absorption bf base fluid eff effective inlet inlet of test section nf nanofluid outlet outlet of test section
In this study, we tried to enhance the CO2 absorption rate to save the energy to maintain the low temperature of the methanol absorbent. The particles (i.e. SiO2, Al2O3 nanoparticles in this study) and methanol are combined into SiO2/methanol, Al2O3/methanol nanofluids to enhance the CO2 absorption rate of the base fluid, which is methanol. The parametric analysis on the effects of particle fraction and nanofluid temperature on CO2 absorption rate is carried out. The maximum CO2 absorption enhancements compared to the pure methanol are obtained w3.1% at 0.01 vol% of Al2O3/methanol nanofluids, w2.8% at 0.05 vol% of SiO2/methanol nanofluids in 20 C, respectively. And the maximum CO2 absorption enhancements compared to the pure methanol are obtained w4.5% at 0.01 vol% of Al2O3/methanol nanofluids, w5.6% at 0.01 vol% of SiO2/ methanol nanofluids in 20 C, respectively. The methanolbased nanofluids are expected to be a promising working fluid material for removing the acid gases.
2.
Experiment
2.1.
Experimental apparatus and procedures
Fig. 1 shows the schematic of experimental apparatus. Initially, the methanol nanofluid is filled in the test section which is a cylindrical type. The temperature of test solution is
Fig. 1 e Schematic diagram of the experimental apparatus.
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controlled by a thermostat and checked by the thermocouple of which measurement error is 0.1 C. The CO2 vapor is introduced from the bottom of the test section by a nozzle with 4 holes. The nozzle is used to produce a minimum bubble size at the bottom of the test section. After then, the bubbles freely rise up within the solution pool and they are absorbed into the solution. The amount of feed gas is controlled by the regulator at the outlet of CO2 tank and the gas flow rates are obtained by the two mass flow meters at the inlet and outlet of test section. Silica gel is used to remove the methanol vapor existing in the outlet of the test section. The nanofluids are prepared by the following two-step method. First, Al2O3 (supplied by the Alfa Aesar) and SiO2 (supplied by the SigmaeAldrich Inc.) nanoparticles are prepared. Fig. 2(a) and (b) shows the FE-SEM images of Al2O3 and SiO2, respectively. Second, the particles are dispersed into the base fluid with ultrasonication of 1 h to form the stable fluid. The particle concentrations of the nanofluid range from 0.005 to 0.5 vol% of nanoparticles. Table 1 shows the details of the experimental conditions. To evaluate the dispersion stability of nanofluids, the hydrodynamic diameters of the particles are measured by the dynamic light scattering method (ELS-Z, Otsuka Elec., JPN) and the dispersion stability of nanofluids is visualized in vials for 24 h. Fig. 3 shows the volume distribution of the nanoparticle size within the nanofluids. Mean particle size of the nanofluid is measured by a dynamic light scattering (DLS) device (ELS-Z, Otsuka, JPN). The particle size is calculated by using the autocorrelation function with measurement of the tremor of the scattering
Table 1 e Experimental conditions. CO2 Purity Inlet mass flux Inlet pressure
99.999% 3.05 105 kg s1 103 kPa
Methanol Purity Temperature
99.8% 20 C, 20 C
Particle size SiO2 Ai2O3 Concentration
10e20 nm 40e50 nm 0.005e0.5 vol%
Test section Diameter Height Inner pressure
70 mm 260 mm 101.3 kPa
Ultrasonication Time Power Frequency
1h 750 W 20 kHz
intensity with HeeNe and Ar lasers. The wave length of the HeeNe and Ar lasers ranges 3 nm w5 mm and 1.4 nm w5 mm, respectively. The measurements of particle size are carried out 10 times for each case. The particle size has the Gaussian distribution so the error bars in the size measurement mean the minimum (lower bar) and maximum (upper bar) sizes of the particles, respectively.
2.2.
Data reduction
The experimental data of the pressure, temperature and gas flow rate are obtained every 2 s by the Vee program (Agilent Tech.). The CO2 absorption rates are obtained using Eq. (1), Z _ abs ¼ m
_ inlet Dt m
Z
_ outlet Dt m
Dt
:
(1)
_ outlet are the gas flow rate at the inlet and the _ inlet and m Where m outlet of the test section and Dt is the absorption time period, respectively. The effective absorption ratio, Reff is defined as the ratio of the absorption rate of the nanofluids to that of the base fluid (pure methanol), which is expressed as Reff ¼
_ abs;nf m : _ abs;bf m
(2)
_ abs;bf and m _ abs;nf are the absorption rates for the base Where m fluid and that for nanofluids, respectively. The physical meaning of Reff is the effectiveness of the nanofluid compared with the base fluid.
Fig. 2 e FE-SEM images of Al2O3, SiO2 nanoparticles; (a) Al2O3, (b) SiO2.
3.
Results and discussion
3.1.
Dispersion stability
Fig. 4 shows the photos of the prepared nanofluids for each particle species, concentration and time. It is found that the
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Fig. 3 e Volume distribution of nanoparticle size within the nanofluids; (a) 0.01 vol% of Al2O3/methanol, (b) 0.05 vol% of SiO2/ methanol.
particles in the SiO2/methanol nanofluids and the Al2O3/ methanol nanofluids are relatively stable for 24 h. However, nanofluids in upper part of vials are found to be separated for 0.05e0.5 vol%. The reason is not only the aggregation but also the sedimentation of nanoparticles at high concentration.
3.2.
Particle size
Figs. 5 and 6 show the particle size variations for each concentration and time for Al2O3/methanol and SiO2/methanol, respectively. The average particle sizes of SiO2/methanol
and Al2O3/methanol are about 340 nm and 200 nm, respectively. The circular symbol is for the particle sizes right after the preparation, and the rectangular one is for 24 h after the preparation. After 24 h, the particle sizes of SiO2/methanol are similar for all concentrations while the particle sizes of Al2O3/ methanol suddenly decrease at 0.5 vol%. Therefore, it is considered that Al2O3 particles are sedimented at 0.5 vol%. It matches well with the visual observation in Fig. 4. The particle size has the Gaussian distribution so the error bar in Figs. 5 and 6 mean the minimum (lower bar) and maximum (upper bar) sizes of particle, respectively.
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Fig. 4 e The photos of the prepared nanofluids for each particle species, concentrations and time.
3.3.
Effective absorption ratio
Figs. 7 and 8 show the absorption rates of pure methanol and nanofluids at 20 C and 20 C, respectively. It is shown that the absorption rates for the nanofluids are higher than those for the pure methanol for both cases. Fig. 9 shows the effective absorption ratios for each particle species and concentration at 20 C. The maximum CO2 absorption enhancements compared to the pure methanol are obtained 3.1% at 0.01 vol% of Al2O3/methanol nanofluids, and 2.8% at 0.05 vol% of SiO2/ methanol nanofluids, respectively. Up to the 0.01 vol% Al2O3 and 0.05 vol% SiO2 particles in the absorbents, the effective absorption ratios increase with increasing the particle
concentration. However, after that point, the effective absorption ratios are inversely proportional to particle concentration. This is because the particles in the nanofluids contribute to break the bubble size, and mix the bubbles and absorbent at a low concentration of particle. After a critical concentration between 0.01 vol% and 0.05 vol% for Al2O3 and SiO2 nanoparticles, the nanoparticles become too dense leading to a negative effect. This is because the velocity disturbance field is reduced by the nanoparticle aggregation. The velocity disturbance field in the fluid, created by the motion of the nanoparticles, enhances the mass diffusion. (Krishnamurthy et al., 2006; Fang et al., 2009) However, the aggregation and sedimentation of nanoparticles at a high
400
400
0 hour 24 hours
300
Size (nm)
Size (nm)
300
200
200 0 hour 24 hours
100
100
0.01
0.1
Concentration (vol%) Fig. 5 e Particle size of Al2O3/methanol for each concentration and time.
1
0.01
0.1
Concentration (vol %) Fig. 6 e Particle size of SiO2/methanol for each concentration and time.
1
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Fig. 7 e Absorption rates of pure methanol and nanofluids at 20 C.
Fig. 8 e Absorption rates of pure methanol and nanofluids at L20 C.
Fig. 9 e Effective absorption ratios for each particle species and concentration at 20 C.
Fig. 10 e Effective absorption ratios for each particle species and concentration at L20 C.
concentration make the density of disturbance field to be reduced. The hydroxyl group (eOH) in methanol (CH3OH) captures CO2 gas at a low concentration of nanoparticles, however, the hydroxyl group bonds with the nanoparticles after the critical concentration during the manufacturing of the nanofluid. Therefore it makes the size of the particles in nanofluid increases, leading to the reduction of the effective absorption ratio. Fig. 10 shows the effective absorption ratios for each particle species and concentration at 20 C. The maximum CO2 absorption enhancements compared to the pure methanol are obtained 4.5% at 0.01 vol% of Al2O3/methanol nanofluids and 5.6% at 0.01 vol% of SiO2/methanol nanofluids, respectively. Likewise, this result shows a similar tendency at 20 C experiment. It is also found that the effective absorption ratios at 20 C are higher than those at 20 C. This is because of high solubility and low bonding force between the nanoparticles and methanol at a lower temperature.
Fig. 11 e pH variation for each particle species and concentration.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
3.4.
pH
Fig. 11 shows the pH variation for each particle species and concentration. The pH decreases with increasing the particle concentration. Normally, the pH of pure methanol is about 8. Adding nanoparticles in methanol impacts on pH value because of particle’s dissociation and bonding with the hydroxyl group. However, metal oxide is not dissociated well at a low temperature and, therefore it is expected that pH values at 20 C are higher than those at 20 C. This is why the effective absorption ratios at 20 C are higher than those at 20 C as shown in Figs. 7 and 8. Therefore it can be concluded that the pH variation is closely related with the absorption enhancement.
4.
Conclusions
In this study, the dispersion stability is evaluated for Al2O3/ methanol and SiO2/methanol nanofluids for each concentration. The bubble absorption experiment is also carried out for the nanofluids. The following conditions are drawn from the present study. 1) In the cases of Al2O3 and SiO2 nanoparticles, the nanofluids are prepared by the ultrasonic treatment and show a good stability. However, it is found that Al2O3 particles are sedimented at 0.5 vol%. 2) It is found that the optimum concentration of Al2O3 and SiO2 nanoparticles ranges 0.01w0.05 vol% in the methanolbased nanofluids 3) It is found that the CO2 absorption rate is enhanced up to 4.5% at 0.01 vol% of Al2O3/methanol nanofluids at 20 C, and 5.6% at 0.01 vol% of SiO2/methanol nanofluids at 20 C, respectively. It is also found that the effective absorption ratios at 20 C are higher than those at 20 C. 4) It is found that the pH variation is closely related with the absorption enhancement by the particles dissociation, and bonding of hydroxyl group and nanoparticles in methanolbased nanofluids.
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Acknowledgment This work was supported by the National Research Foundation (NRF) grant (No. 20100029120).
references
Ahmad, A.L., Sunarti, A.R., Lee, K.T., Fernando, W.J.N., 2010. CO2 removal using membrane gas absorption. Int. J. Greenhouse Gas Control 4, 495e498. Chang, H.M., Chung, M.J., Park, S.B., 2009. Cryogenic heatexchanger design for freeze-out removal of carbon dioxide from landfill gas. J. Therm. Sci. Technol. 4, 362e371. Dennis, J.S., Scott, S.A., 2010. In situ gasification of a lignite coal and CO2 separation using chemical looping with a Cu-based oxygen carrier. Fuel 89, 1623e1640. Fang, X., Xuan, Y., Li, Q., 2009. Experimental investigation on enhanced mass transfer in nanofluids. Appl. Phys. Lett. 95 (20), 203108. Figueroa, J.D., Fout, T., Plasynski, S., McIlvried, H., Srivastava, R.D., 2008. Advances in CO2 capture technology e The U.S. Department of Energy’s Carbon Sequestration Program. Int. J. Greenhouse Gas Control 2, 9e20. Kim, W.G., Kang, H.U., Jung, K.M., Kim, S.H., 2008. Synthesis of silica nanofluid and application to CO2 absorption. Sep. Sci. Technol. 43, 3036. Krishnamurthy, S., Bhattacharya, P., Phelan, P.E., Prasher, R.S., 2006. Enhanced mass transport in nanofluids. Nano Lett. 6 (3), 419e423. Korens, G., Simbeck, D.R., Wilhelm, D.J., 2002. Process screening analysis of alternative gas treating and sulfur removal for gasification, Prepared for U.S. Department of Energy by SFA Pacific, Inc., Revised Final Report. Ranke, G., Mohr, V.H., 1985. The Rectisol wash: new developments in acid gas removal from synthesis gas. In: Newman, Stephen A. (Ed.), Acid and Sour Gas Treating Processes. Gulf Publishing Company, Houston, pp. 80e111. Zhang, Z.J., Zhang, W., Chen, X., Xia, Q.B., Li, Z., 2010. Adsorption of CO2 on zeolite 13X and activated carbon with higher surface area. Sep. Sci. Technol. 45, 710e719.
available at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
CO2 bubble absorption enhancement in methanol-based nanofluids Jae Won Lee a, Jung-Yeul Jung b, Soon-Geul Lee a, Yong Tae Kang a,* a
Department of Mechanical Engineering, Kyung Hee University, Yong In, Gyeong-gi 446-701, Republic of Korea Marine Safety & Pollution Response Research Department, Maritime & Ocean Engineering Research Institute, KORDI, Daejon 305-343, Republic of Korea b
article info
abstract
Article history:
In this study, the nanoparticles (i.e. SiO2 and Al2O3 nanoparticles) and methanol are
Received 21 April 2011
combined into SiO2/methanol and Al2O3/methanol nanofluids to enhance the CO2
Received in revised form
absorption rate of the base fluid (methanol). The absorption experiments are performed in
16 July 2011
the bubble type absorber system equipped with mass flow controller (MFC), mass flow
Accepted 9 August 2011
meter (MFM) and silica gel (which can remove the methanol vapor existing in the outlet
Available online 16 August 2011
gases). The parametric analysis on the effects of the particle species and concentrations on CO2 bubble absorption rate is carried out. The particle concentration ranges from 0.005 to
Keywords:
0.5 vol%. It is found that the CO2 absorption rate is enhanced up to 4.5% at 0.01 vol% of
Absorption
Al2O3/methanol nanofluids at 20 C, and 5.6% at 0.01 vol% of SiO2/methanol nanofluids at
Enhancement
20 C, respectively. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Bubble Carbon dioxide Methanol Particle
Ame´lioration de l’absorption des bulles de CO2 dans les nanofluides a` base de me´thanol Mots cle´s : Absorption ; Ame´lioration ; Bulle ; Dioxyde de carbone ; Me´thanol ; Particule
1.
Introduction
The shortage of oil will increase the prices of the crude oil, petrochemicals and natural gas in near future. So the synthetic natural gas (SNG) from the coal is one of the promising alternative energy resources. An integrated gasification combined cycle (IGCC) is a power plant system using
the synthesis gas (syngas). In the IGCC, it is required to remove the acid gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S) from valuable feed gas streams. By doing so, the feed gas is made more suitable for combustion and further processing. The ways to remove the CO2 are absorption, adsorption, membranes, cryogenics, chemical looping, etc. (Figueroa
* Corresponding author. Tel.: þ82 31 201 2990; fax: þ82 31 202 3260. E-mail address: [email protected] (Y.T. Kang). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.08.002
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
Nomenclature MFC MFM _ m R t
mass flow controller mass flow meter mass flow rate, kg s1 absorption ratio absorption time, s
et al., 2008; Kim et al., 2008; Chang et al., 2009; Ahmad et al., 2010; Dennis and Scott, 2010; Zhang et al., 2010) So far, the methods of membranes, cryogenics, chemical looping, etc., have not been developed to a practical stage. Currently, the most used ways to remove CO2 are absorption and adsorption methods. However, the adsorption method needs too much energy for desorption process and is not proper for large-scale systems. The absorption methods are divided into physical and chemical types according to the types of absorbents. The chemical absorption type is suitable for atmospheric pressure. Therefore, this type of absorption is suitable to remove CO2 in the exhaust gas. Nonetheless, there are problems such as degradation of the absorbent and the need of much energy for regeneration. However, the physical absorption method is used in the IGCC system due to its suitability for high pressure. Recently, Rectisol system, that is physical absorption type, is used prevalently to remove the acid gases in the SNG production system. This system uses methanol as an absorbent. The advantages of methanol absorbent are cheap and selective absorption of acid gases such as CO2, H2S and COS (Korens et al., 2002; Ranke and Mohr, 1985). In the Rectisol system, the temperature of methanol absorbent should be maintained at least about 20 C to increase the absorption rate according to the Henry’s solubility law. Therefore, huge amount of energy is required to keep such a low temperature of methanol in the Rectisol system.
Subscripts abs absorption bf base fluid eff effective inlet inlet of test section nf nanofluid outlet outlet of test section
In this study, we tried to enhance the CO2 absorption rate to save the energy to maintain the low temperature of the methanol absorbent. The particles (i.e. SiO2, Al2O3 nanoparticles in this study) and methanol are combined into SiO2/methanol, Al2O3/methanol nanofluids to enhance the CO2 absorption rate of the base fluid, which is methanol. The parametric analysis on the effects of particle fraction and nanofluid temperature on CO2 absorption rate is carried out. The maximum CO2 absorption enhancements compared to the pure methanol are obtained w3.1% at 0.01 vol% of Al2O3/methanol nanofluids, w2.8% at 0.05 vol% of SiO2/methanol nanofluids in 20 C, respectively. And the maximum CO2 absorption enhancements compared to the pure methanol are obtained w4.5% at 0.01 vol% of Al2O3/methanol nanofluids, w5.6% at 0.01 vol% of SiO2/ methanol nanofluids in 20 C, respectively. The methanolbased nanofluids are expected to be a promising working fluid material for removing the acid gases.
2.
Experiment
2.1.
Experimental apparatus and procedures
Fig. 1 shows the schematic of experimental apparatus. Initially, the methanol nanofluid is filled in the test section which is a cylindrical type. The temperature of test solution is
Fig. 1 e Schematic diagram of the experimental apparatus.
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controlled by a thermostat and checked by the thermocouple of which measurement error is 0.1 C. The CO2 vapor is introduced from the bottom of the test section by a nozzle with 4 holes. The nozzle is used to produce a minimum bubble size at the bottom of the test section. After then, the bubbles freely rise up within the solution pool and they are absorbed into the solution. The amount of feed gas is controlled by the regulator at the outlet of CO2 tank and the gas flow rates are obtained by the two mass flow meters at the inlet and outlet of test section. Silica gel is used to remove the methanol vapor existing in the outlet of the test section. The nanofluids are prepared by the following two-step method. First, Al2O3 (supplied by the Alfa Aesar) and SiO2 (supplied by the SigmaeAldrich Inc.) nanoparticles are prepared. Fig. 2(a) and (b) shows the FE-SEM images of Al2O3 and SiO2, respectively. Second, the particles are dispersed into the base fluid with ultrasonication of 1 h to form the stable fluid. The particle concentrations of the nanofluid range from 0.005 to 0.5 vol% of nanoparticles. Table 1 shows the details of the experimental conditions. To evaluate the dispersion stability of nanofluids, the hydrodynamic diameters of the particles are measured by the dynamic light scattering method (ELS-Z, Otsuka Elec., JPN) and the dispersion stability of nanofluids is visualized in vials for 24 h. Fig. 3 shows the volume distribution of the nanoparticle size within the nanofluids. Mean particle size of the nanofluid is measured by a dynamic light scattering (DLS) device (ELS-Z, Otsuka, JPN). The particle size is calculated by using the autocorrelation function with measurement of the tremor of the scattering
Table 1 e Experimental conditions. CO2 Purity Inlet mass flux Inlet pressure
99.999% 3.05 105 kg s1 103 kPa
Methanol Purity Temperature
99.8% 20 C, 20 C
Particle size SiO2 Ai2O3 Concentration
10e20 nm 40e50 nm 0.005e0.5 vol%
Test section Diameter Height Inner pressure
70 mm 260 mm 101.3 kPa
Ultrasonication Time Power Frequency
1h 750 W 20 kHz
intensity with HeeNe and Ar lasers. The wave length of the HeeNe and Ar lasers ranges 3 nm w5 mm and 1.4 nm w5 mm, respectively. The measurements of particle size are carried out 10 times for each case. The particle size has the Gaussian distribution so the error bars in the size measurement mean the minimum (lower bar) and maximum (upper bar) sizes of the particles, respectively.
2.2.
Data reduction
The experimental data of the pressure, temperature and gas flow rate are obtained every 2 s by the Vee program (Agilent Tech.). The CO2 absorption rates are obtained using Eq. (1), Z _ abs ¼ m
_ inlet Dt m
Z
_ outlet Dt m
Dt
:
(1)
_ outlet are the gas flow rate at the inlet and the _ inlet and m Where m outlet of the test section and Dt is the absorption time period, respectively. The effective absorption ratio, Reff is defined as the ratio of the absorption rate of the nanofluids to that of the base fluid (pure methanol), which is expressed as Reff ¼
_ abs;nf m : _ abs;bf m
(2)
_ abs;bf and m _ abs;nf are the absorption rates for the base Where m fluid and that for nanofluids, respectively. The physical meaning of Reff is the effectiveness of the nanofluid compared with the base fluid.
Fig. 2 e FE-SEM images of Al2O3, SiO2 nanoparticles; (a) Al2O3, (b) SiO2.
3.
Results and discussion
3.1.
Dispersion stability
Fig. 4 shows the photos of the prepared nanofluids for each particle species, concentration and time. It is found that the
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Fig. 3 e Volume distribution of nanoparticle size within the nanofluids; (a) 0.01 vol% of Al2O3/methanol, (b) 0.05 vol% of SiO2/ methanol.
particles in the SiO2/methanol nanofluids and the Al2O3/ methanol nanofluids are relatively stable for 24 h. However, nanofluids in upper part of vials are found to be separated for 0.05e0.5 vol%. The reason is not only the aggregation but also the sedimentation of nanoparticles at high concentration.
3.2.
Particle size
Figs. 5 and 6 show the particle size variations for each concentration and time for Al2O3/methanol and SiO2/methanol, respectively. The average particle sizes of SiO2/methanol
and Al2O3/methanol are about 340 nm and 200 nm, respectively. The circular symbol is for the particle sizes right after the preparation, and the rectangular one is for 24 h after the preparation. After 24 h, the particle sizes of SiO2/methanol are similar for all concentrations while the particle sizes of Al2O3/ methanol suddenly decrease at 0.5 vol%. Therefore, it is considered that Al2O3 particles are sedimented at 0.5 vol%. It matches well with the visual observation in Fig. 4. The particle size has the Gaussian distribution so the error bar in Figs. 5 and 6 mean the minimum (lower bar) and maximum (upper bar) sizes of particle, respectively.
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Fig. 4 e The photos of the prepared nanofluids for each particle species, concentrations and time.
3.3.
Effective absorption ratio
Figs. 7 and 8 show the absorption rates of pure methanol and nanofluids at 20 C and 20 C, respectively. It is shown that the absorption rates for the nanofluids are higher than those for the pure methanol for both cases. Fig. 9 shows the effective absorption ratios for each particle species and concentration at 20 C. The maximum CO2 absorption enhancements compared to the pure methanol are obtained 3.1% at 0.01 vol% of Al2O3/methanol nanofluids, and 2.8% at 0.05 vol% of SiO2/ methanol nanofluids, respectively. Up to the 0.01 vol% Al2O3 and 0.05 vol% SiO2 particles in the absorbents, the effective absorption ratios increase with increasing the particle
concentration. However, after that point, the effective absorption ratios are inversely proportional to particle concentration. This is because the particles in the nanofluids contribute to break the bubble size, and mix the bubbles and absorbent at a low concentration of particle. After a critical concentration between 0.01 vol% and 0.05 vol% for Al2O3 and SiO2 nanoparticles, the nanoparticles become too dense leading to a negative effect. This is because the velocity disturbance field is reduced by the nanoparticle aggregation. The velocity disturbance field in the fluid, created by the motion of the nanoparticles, enhances the mass diffusion. (Krishnamurthy et al., 2006; Fang et al., 2009) However, the aggregation and sedimentation of nanoparticles at a high
400
400
0 hour 24 hours
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Size (nm)
Size (nm)
300
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200 0 hour 24 hours
100
100
0.01
0.1
Concentration (vol%) Fig. 5 e Particle size of Al2O3/methanol for each concentration and time.
1
0.01
0.1
Concentration (vol %) Fig. 6 e Particle size of SiO2/methanol for each concentration and time.
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
Fig. 7 e Absorption rates of pure methanol and nanofluids at 20 C.
Fig. 8 e Absorption rates of pure methanol and nanofluids at L20 C.
Fig. 9 e Effective absorption ratios for each particle species and concentration at 20 C.
Fig. 10 e Effective absorption ratios for each particle species and concentration at L20 C.
concentration make the density of disturbance field to be reduced. The hydroxyl group (eOH) in methanol (CH3OH) captures CO2 gas at a low concentration of nanoparticles, however, the hydroxyl group bonds with the nanoparticles after the critical concentration during the manufacturing of the nanofluid. Therefore it makes the size of the particles in nanofluid increases, leading to the reduction of the effective absorption ratio. Fig. 10 shows the effective absorption ratios for each particle species and concentration at 20 C. The maximum CO2 absorption enhancements compared to the pure methanol are obtained 4.5% at 0.01 vol% of Al2O3/methanol nanofluids and 5.6% at 0.01 vol% of SiO2/methanol nanofluids, respectively. Likewise, this result shows a similar tendency at 20 C experiment. It is also found that the effective absorption ratios at 20 C are higher than those at 20 C. This is because of high solubility and low bonding force between the nanoparticles and methanol at a lower temperature.
Fig. 11 e pH variation for each particle species and concentration.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 7 2 7 e1 7 3 3
3.4.
pH
Fig. 11 shows the pH variation for each particle species and concentration. The pH decreases with increasing the particle concentration. Normally, the pH of pure methanol is about 8. Adding nanoparticles in methanol impacts on pH value because of particle’s dissociation and bonding with the hydroxyl group. However, metal oxide is not dissociated well at a low temperature and, therefore it is expected that pH values at 20 C are higher than those at 20 C. This is why the effective absorption ratios at 20 C are higher than those at 20 C as shown in Figs. 7 and 8. Therefore it can be concluded that the pH variation is closely related with the absorption enhancement.
4.
Conclusions
In this study, the dispersion stability is evaluated for Al2O3/ methanol and SiO2/methanol nanofluids for each concentration. The bubble absorption experiment is also carried out for the nanofluids. The following conditions are drawn from the present study. 1) In the cases of Al2O3 and SiO2 nanoparticles, the nanofluids are prepared by the ultrasonic treatment and show a good stability. However, it is found that Al2O3 particles are sedimented at 0.5 vol%. 2) It is found that the optimum concentration of Al2O3 and SiO2 nanoparticles ranges 0.01w0.05 vol% in the methanolbased nanofluids 3) It is found that the CO2 absorption rate is enhanced up to 4.5% at 0.01 vol% of Al2O3/methanol nanofluids at 20 C, and 5.6% at 0.01 vol% of SiO2/methanol nanofluids at 20 C, respectively. It is also found that the effective absorption ratios at 20 C are higher than those at 20 C. 4) It is found that the pH variation is closely related with the absorption enhancement by the particles dissociation, and bonding of hydroxyl group and nanoparticles in methanolbased nanofluids.
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Acknowledgment This work was supported by the National Research Foundation (NRF) grant (No. 20100029120).
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