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CO2 absorption enhancement by methanol-based Al2O3 and SiO2 nanofluids in a tray column absorber
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
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CO2 absorption enhancement by methanol-based Al2O3 and SiO2 nanofluids in a tray column absorber Israel Torres Pineda, Jae Won Lee, Inhwa Jung, Yong Tae Kang*,1 Department of Mechanical Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea
article info
abstract
Article history:
In the present study, suspensions of Al2O3 and SiO2 nanoparticles in methanol (nanofluid) are
Received 9 January 2012
produced and analyzed for the application of CO2 absorption in a tray column absorber. The
Received in revised form
absorber is an acrylic tray column with twelve plates. The column is a sieve tray type which
27 February 2012
has flat perforated plates where the vapor velocity keeps the liquid from flowing down through
Accepted 27 March 2012
the holes and the CO2 gas and methanol liquid are brought in contact in a counter-current
Available online 4 April 2012
flow. The test section is equipped with two mass flow meters to measure the absorption
Keywords:
SiO2 particles (compared to pure methanol), respectively. It is also found that SiO2 nano-
Absorption
particle is a better candidate than Al2O3 nanoparticle and 0.05 vol% of nanoparticles is an
Enhancement
optimum condition for CO2 absorption enhancement for the present experimental conditions.
rate. The results show maximum enhanced absorption rates of 9.4% and 9.7% for Al2O3 and
ª 2012 Elsevier Ltd and IIR. All rights reserved.
Bubbles Carbon dioxide Continuous flow Methanol Particle
Ame´lioration de l’absorption du CO2 a` l’aide de nanofluides a` base de me´thanol au Al2O3 et au SiO2 dans un absorbeur a` colonne a` plateaux Mots cle´s : Absorption ; Ame´lioration ; Bulles ; Dioxyde de carbone ; E´coulement continu ; Me´thanol e Particule
1.
Introduction
Synthetic natural gas (SNG) is used in an integrated gasification combined cycle (IGCC) as fuel for combustion with the final purpose of producing energy. In an IGCC plant, acid gases such
as carbon dioxide (CO2) and hydrogen sulphide (H2S) need to be removed from the valuable feed gas streams. The acid gas removal (AGR) system produces a more suitable feed gas for combustion and further processing. This energy production technology is coming to the spot line as a consequence of the
* Corresponding author. Tel.: þ82 31 201 2990; fax: þ82 31 202 3260. E-mail address: [email protected] (Y.T. Kang). 1 IIR B1 Session, President. 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.03.017
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
Standard deviation, g s1
Nomenclature
s
B _ m
Subscripts abs absorption bf base fluid eff effective inlet inlet of test section nf nanofluid outlet outlet of test section
N R S t U w
Systematic uncertainty, % Mass flow rate, g s1 Number of measurements Absorption ratio Random uncertainty, % Absorption time, s Overall uncertainty, % Uncertainty, g s1
worldwide increase in the prices of crude oil and petrochemicals due to the oil scarcity in present days. Therefore, much attention is being paid to different alternatives such as SNG, which is becoming one of the promising options for energy production. The Rectisol process, developed by Lurgi GmbH, is a physical absorption process widely used to remove the acid gases in the IGCC. In the Rectisol process, the methanol absorbent should be cooled down to a temperature of about 40 C at normal operational conditions. The need to refrigerate the solvent results in a high capital and operating cost and is one of the main disadvantages of this process (Korens et al., 2002). However, methanol offers the highest CO2 absorption capacity relative to other solvents (Esteban et al., 2000). Therefore, the purpose of this study is to enhance the CO2 absorption rate of methanol with the use of Al2O3 and SiO2 nanoparticles. Recent studies have shown an improvement in CO2 absorption by using nanofluids. Kim et al. (2008) developed silica nanofluids in water for application in CO2 absorption; the results showed a 79% of increase in CO2 absorption rate and a 24% increase in total absorption in a bubble type absorber. In addition, experiments in a bubble type absorber were carried out by Lee et al. (2011) with methanol-based fluids with alumina and silica particles at different concentrations. It was
1403
concluded that the maximum CO2 absorption enhancements compared to the pure methanol were 4.5% at 0.01 vol% of Al2O3 and 5.6% at 0.01 vol% of SiO2, respectively. In the present study, suspensions of Al2O3 and SiO2 nanoparticles in methanol (nanofluid) are produced and analyzed for the application of CO2 absorption in a tray column absorber. The objectives of this study are to find an optimum concentration of nanoparticles, and to determine the absorption performance enhancement by using the methanol-based nanofluids in a tray column absorber for the given geometrical and operational conditions, which has a completely different absorption mechanism from the bubble type absorber. A model for the absorption enhancement is also proposed, the bubble breaking model, which is based on the absorption experiments and the bubble visualization.
2.
Experiments
2.1. Experimental apparatus for the absorption experiment Fig. 1 shows the schematic diagram of the experimental apparatus. The continuous removal system is an acrylic tray
Fig. 1 e Schematic of the experimental apparatus.
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2.3.
Table 1 e Geometrical conditions. Number of plates Total length [cm] Diameter Dc [cm] Tray spacing [cm] Area [cm2] Net area [cm2] Active area [cm2] Hole area [cm2] Vol. per plate [cm3] Hole diameter [cm]
12 78 3 5 7.07 6.22 5.37 0.54 3.11 0.1
column with twelve plates. The column is a sieve tray type and has flat perforated plates where the vapor velocity keeps the liquid from flowing down through the holes. Table 1 shows the geometric conditions of the tray column. The column is equipped with an oval gear flow meter (accuracy 1% of reading) to measure the flow rate of the absorbent. Two gas flow meters with the same characteristics measure the flow of CO2 gas at the inlet and outlet (accuracy 1% of full scale). A desiccant unit is located before the outlet CO2 flow meter to remove methanol from the CO2 flow. Four thermocouples (accuracy 1.5 C) are located at the outlet and inlet of the gas and liquid to measure the temperatures of both the gas and the liquid.
2.2.
Preparation of the nanofluids
Inlet conditions are listed in Table 2. The fluid flow is increased slowly until the desired mass flow rate value is reached. The absorbent enters the tray column and flows downwards within the test section. The CO2 gas is measured by a mass flow meter; it enters from the bottom and goes upward as shown in Fig. 1. In each plate, the CO2 gas keeps the liquid from flowing down through the holes, then the gas passes through the perforations and the bubbles form a froth, disengages from the froth and passes on to the next tray above as shown in Fig. 2. During the time the liquid and the gas are in contact, interphase diffusion occurs and the methanol with the absorbed CO2 flows out of the system from the bottom of the tower into a tank. On the other side, the remaining CO2 gas flows out of the system from the top of the tower and passes through the outlet mass flow meter to be measured.
2.4.
Data reduction
Data is transferred from the data acquisition unit to the dataflow program (VEE, Agilent Technologies) every 2 s for a total of 400 s. However, it takes some time to stabilize the flow rate of the liquid and the gas, therefore, we take 100 s of stable behavior for our calculations. The CO2 absorption rate and the effective absorption ratio which is defined as the ratio of the absorption rate of the nanofluids to that of the pure methanol are obtained by Eqs. (1) and (2), respectively. Z
The nanofluid absorbents are prepared with aluminum oxide (Al2O3 99.5%, 40e50 nm, Alfa Aesar) and silicon dioxide (SiO2 99.5%, 10e20 nm, SigmaeAldrich) nanoparticles suspended in methanol (base fluid). The particles are stabilized into the methanol using an ultra-sonicator. With the application of sonication, there is no need to use surfactants to obtain a good dispersion of the nanoparticles in methanol. The solution is continuously circulated and sonicated for 1 h inside a flow cell reactor.
_ abs ¼ m
Reff ¼
Table 2 e Operational inlet conditions for the tray column absorber. CO2 Purity [%] Inlet flux [kg s1] Inlet Pressure [kPa]
99.999 6.66 105 104 10
Methanol Purity [%] Inlet flux [kg s1] Temperature [ C]
99.8 0.145 22 1
Al2O3 Size [nm] Concentration [vol%]
40e50 0.005, 0.01, 0.05, 0.1
_ inlet dt m
Z
_ outlet dt m
Dt
_ abs;nf m _ abs;bf m
(1)
(2)
The physical meaning of the effective absorption ratio is the effectiveness of the nanofluids for CO2 absorption enhancement.
2.5.
SiO2 Size [nm] Concentration [vol%]
Procedures for the absorption experiment
Experimental error analysis
The experimental error analysis was performed based on Holman (2001). The experimental uncertainty is determined by calculating the uncertainty in the results based on the systematic uncertainty and the random standard uncertainty. The random uncertainty is calculated based on the standard deviation of the mean (SDOM). The experimental uncertainties ranged 1.6%e10.2% as shown in Table 3. Uε ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 B2ε þ S2ε
where, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ outlet $woutlet Þ2 _ inlet $winlet Þ2 þðm ðm Bε ¼ _ abs m
(3)
(4)
and, 10e20 0.005, 0.01, 0.05, 0.1
2sabs Sε ¼ pffiffiffiffi _ abs N$m
(5)
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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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
Fig. 2 e Pictures of the absorption process in a tray for the different concentrations of Al2O3 in methanol.
Table 3 e Experimental error analysis of the experimental results. Concentration [vol%]
Measured CO2 flow [g s1 102]
CO2 flow uncertainty [g s1 103]
Uncertainty [%]
_ abs m
winlet
woutlet
Bε
Sε
Uε
Pure methanol
2.45
4.60
2.76
1.2
1.7
3.0
SiO2 0.005 0.01 0.05 0.1
2.58 2.67 2.68 2.59
4.57 4.57 4.55 4.54
2.63 2.56 2.53 2.60
1.1 1.1 1.1 1.1
2.7 0.9 0.4 1.1
4.2 2.0 1.6 2.3
Al2O3 0.005 0.01 0.05 0.1
2.45 2.53 2.68 2.53
4.56 4.55 4.56 4.56
2.71 2.65 2.55 2.66
1.2 1.1 1.1 1.1
3.2 7.1 2.9 2.0
4.8 10.2 4.4 3.3
3.
Results and discussion
3.1.
Dispersion stabilization and particle size
Fig. 3 shows the prepared nanofluids at different Al2O3 and SiO2 concentrations right after the preparation and 24 h after the preparation. From the visual observations, we can see no
Fig. 3 e Pictures of the prepared nanofluids at different concentrations of SiO2 and Al2O3 right after the preparation and 24 h after.
Fig. 4 e Particle size of the absorbents at different SiO2 concentrations after 24 h.
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Fig. 8 e Bubble breaking model (The nanoparticles cover the bubble and break them into smaller bubbles). Fig. 5 e Particle size of the absorbents at different Al2O3 concentrations after 24 h. Table 4 e Enhancements obtained for the different concentrations of Al2O3 and SiO2. Concentration [vol%]
Fig. 6 e Absorption rate of the nanofluids prepared with Al2O3.
Fig. 7 e Absorption rate of the nanofluids prepared with SiO2.
Enhancement [%]
Al2O3 0.005 0.01 0.05 0.1
0.2 3.4 9.4 3.5
SiO2 0.005 0.01 0.05 0.1
5.6 9.0 9.7 5.9
clear sign of sedimentation for the concentrations of 0.005 and 0.01 vol% for both particles. To verify the stabilization of the prepared nanofluids, the size of the nanoparticles were measured by dynamic light scattering (DLS). Figs. 4 and 5 show the particle sizes at different concentrations for the absorbents prepared with Al2O3 and SiO2 respectively. The
Fig. 9 e Effective absorption ratio at different concentrations of Al2O3 and SiO2.
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
absorbents with concentrations from 0.005 to 0.1 vol% show similar values after 24 h, confirming the good stability of the solutions. However, it is known that for higher concentrations of both Al2O3 and SiO2 (e.g. 0.5vol%), the particle size measurements after 24 h show a smaller particle size (Lee
1407
et al., 2011), which is a sign of sedimentation and could be caused by the clustering of the particles. It is believed that too much clustering may lead to large regions of “article free” liquid which in the case of heat transfer may exert a negative effect (Keblinski et al., 2002). Therefore, it is concluded that
Fig. 10 e Pictures of CO2 bubbles (500 fps) in methanol and nanofluids at different concentrations of Al2O3 and SiO2. (*) indicates the detached bubbles.
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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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
the concentration ranges between 0.005 and 0.1 vol% of Al2O3 and SiO2 nanoparticles are recommended for good dispersion stability of the methanol-based nanofluids.
3.2.
Absorption rate
The absorption rate, in grams per second, is shown in Fig. 6 for Al2O3 and Fig. 7 for SiO2, respectively. It is found that the absorption rates enhance up to 9.4% for Al2O3 and 9.7% for SiO2 at the critical concentration of 0.05 vol%, after which the absorption rate decreases. Therefore, the critical concentration of 0.05 vol% is considered as an optimum concentration for CO2 absorption enhancement in methanol-based nanofluids with Al2O3 and SiO2 nanoparticles. The mechanism that explains such mass transfer enhancement is still speculative. Some theories such as the hydrodynamic effect propose that the particles may increase the specific interfacial area by covering the bubble surface and preventing the coalescence of the bubbles, resulting in smaller bubbles (Linek et al., 2008). Another possibility is that the particles may collide inducing local turbulence and refreshing the gas-liquid boundary layer by mixing it into the bulk liquid (Ruthiya et al., 2006). However, the general mechanism of the mass transfer enhancement is still unknown. In the work by Krishnamurthy et al. (2006), it is concluded that the Brownian motion of the nanoparticles is not directly responsible for the mass transfer enhancement, but mainly due to the velocity disturbance field in the fluid created by the motion of the nanoparticles. Most of the mass transfer enhancement mechanisms are developed for bubble systems (bubble columns, stirred tanks). However, in a dynamic flow system such as a tray column absorber, the forces induced by the movement of the liquid and vapor are believed to play more dominant roles in the mass transfer enhancement. In the bubble breaking model as shown in Fig. 8, the nanoparticles suspended in the base fluid cover the bubble, and as the movement of the fluid due to external forces become more dynamic, the particles collide with the gas-liquid interface, breaking the bubble into smaller size bubbles. More bubbles mean a larger interfacial area which would promote the mass transfer from the gas to the liquid. Table 4 shows the results of the enhancements achieved by the use of methanol-based nanofluids with SiO2 and Al2O3 nanoparticles. From Table 4, it is concluded that SiO2 nanoparticle is a better candidate for CO2 absorption enhancement than Al2O3 nanoparticles. Fig. 9 shows the effective absorption ratio, which is defined in Eq. (2). The effective absorption ratio increases as the particle concentration increases and both nanoparticles follow a similar trend. After reaching its critical point at 0.05 vol%, both absorbents reduce their enhancement effect and an opposite effect takes place. This behavior has been studied and it has been found that the self-diffusion coefficient decreases as the concentration of nanoparticles, in the liquid phase, increases. Gerardi et al. (2009) reported a decay trend in the diffusion coefficient for Al2O3/water and suggest two effects promoting this behavior: first, the tortuosity of the diffusion path of the water molecules is increased when solid particles stand in their way; second, the water molecules in the ordered layer on the surface of the particles are “bound” and move with the particles, which have a lower diffusion
coefficient than the free molecules. In summary, at low concentrations the movement of the particles promote mass transfer, however after the critical phase (an optimum concentration of 0.05 vol% in the present study), the nanoparticles become too dense in the liquid phase reducing the self-diffusion coefficient and consequently reducing the absorption of the gas phase.
3.3.
Bubble visualization
Additional visualization experiments were performed in order to observe the behavior of the CO2 bubbles in methanol/Al2O3 and methanol/SiO2 nanofluids. A Hele-Shaw cell with a separation of 5 mm between walls conforms the test section. The orifice is a rectangular shape of 5 7 mm. This configuration is applied to make possible the visualization of the nanofluids with the highest concentrations of 0.005 vol% and 0.1 vol%. A syringe pump is utilized to insert the gas into the liquid. Pictures are taken with a high-speed camera at 500 frames per second. From the pictures in Fig. 10, we can observe, in the pictures marked with (*), that the CO2 bubbles with higher concentration detach faster from the orifice. The detachment of the CO2 bubbles in pure methanol occurs at 68 ms while the detachment for SiO2 and Al2O3 nanofluid at 0.1 vol% occurs at 60 ms and 62 ms, respectively. The faster detachment induces a higher frequency rate which is consistent with the experimental work by Liang-Shih Fan et al. (2007). The higher frequency yields a smaller bubble size which promotes the mass transfer to the liquid phase. This result supports the CO2 absorption enhancement model proposed in the present study: the bubble breaking model.
4.
Conclusions
In this study, nanofluids made with Al2O3 and SiO2 nanoparticles at different concentrations in a methanol base fluid are produced and tested in a tray column absorber for CO2 absorption. The results are as follows: 1) It is concluded that the concentration ranges between 0.005 and 0.1 vol% of Al2O3 and SiO2 nanoparticles are recommended for good dispersion stability of the methanolbased nanofluids. 2) With the use of Al2O3 and SiO2 nanofluids in the tray column absorber, we can obtain a higher absorption rate compared with the pure methanol under the same conditions. It is found that the absorption rates of Al2O3 and SiO2 nanofluids enhance up to 9.4% and 9.7% respectively at the critical concentration of 0.05 vol%. It is also found that the SiO2 nanoparticle is a better candidate than Al2O3 nanoparticle and 0.05 vol% of nanoparticles is an optimum condition for CO2 absorption enhancement. 3) It is confirmed, from the visualization results, that the use of Al2O3 and SiO2 in methanol-based nanofluids promotes the detachment of the bubbles form the orifice, resulting in a higher frequency rate and therefore, enhancing the absorption rate.
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
Acknowledgment This work was supported by the National Research Foundation (NRF) grant (No. 20100029120)
references
Esteban, A., Hernandez, V., Lunsford, K., 2000. Exploit the benefits of methanol. In: Proceedings of the 79th GPA Annual Convention. Processors Association, Atlanta, GA. Fan, L.S., Hemminger, O., Yu, Z., Wang, F., 2007. Bubbles in nanofluids. Ind. Eng. Chem. Res. 46, 4341e4346. Gerardi, C., Cory, D., Buongiomo, J., Hu, L.W., McKrell, T., 2009. Nuclear magnetic resonance-based study of ordered layering on the surface of alumina nanoparticles in water. Appl. Phys. Lett. 95, 253104. Holman, J.P., 2001. Experimental Methods for Engineers, seventh ed., International Edition. McGraw Hill. Keblinski, P., Phillpot, S.R., Choi, S.U.S., Eastman, J.A., 2002. Mechanisms of heat flow in suspensions of nano-sized
1409
particles (nanofluids). Int. J. Heat Mass Transfer 45, 855e863. Kim, W.G., Kang, H.U., Jung, K.M., Kim, S.H., 2008. Synthesis of silica nanofluid and application to CO2 absorption. Separat. Sci. Technol. 43, 3036e3055. 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. Krishnamurthy, S., Bhattacharya, P., Phelan, P.E., Prasher, R.S., 2006. Enhanced mass transport in nanofluids. Nano Lett. 6 (3), 419e423. Lee, J.W., Jung, J.Y., Lee, S.G., Kang, Y.T., 2011. CO2 bubble absorption enhancement in methanol-based nanofluids. Int. J. Refrigeration 34 (8), 1727e1733. Linek, V., Kordac, M., Soni, M., 2008. Mechanism of gas absorption enhancement in presence of fine solid particles in mechanically agitated gaseliquid dispersion. Effect of molecular diffusivity. Chem. Eng. Sci. 63, 5120e5128. Ruthiya, K.C., van der Schaaf, J., Kuster, B.F.M., Schouten, J.C., 2006. Influence of particles and electrolyte on gas hold-up and mass transfer in a slurry bubble column. Int. J. Chem. Reactor Eng. 4, A13.
Available online at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
CO2 absorption enhancement by methanol-based Al2O3 and SiO2 nanofluids in a tray column absorber Israel Torres Pineda, Jae Won Lee, Inhwa Jung, Yong Tae Kang*,1 Department of Mechanical Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea
article info
abstract
Article history:
In the present study, suspensions of Al2O3 and SiO2 nanoparticles in methanol (nanofluid) are
Received 9 January 2012
produced and analyzed for the application of CO2 absorption in a tray column absorber. The
Received in revised form
absorber is an acrylic tray column with twelve plates. The column is a sieve tray type which
27 February 2012
has flat perforated plates where the vapor velocity keeps the liquid from flowing down through
Accepted 27 March 2012
the holes and the CO2 gas and methanol liquid are brought in contact in a counter-current
Available online 4 April 2012
flow. The test section is equipped with two mass flow meters to measure the absorption
Keywords:
SiO2 particles (compared to pure methanol), respectively. It is also found that SiO2 nano-
Absorption
particle is a better candidate than Al2O3 nanoparticle and 0.05 vol% of nanoparticles is an
Enhancement
optimum condition for CO2 absorption enhancement for the present experimental conditions.
rate. The results show maximum enhanced absorption rates of 9.4% and 9.7% for Al2O3 and
ª 2012 Elsevier Ltd and IIR. All rights reserved.
Bubbles Carbon dioxide Continuous flow Methanol Particle
Ame´lioration de l’absorption du CO2 a` l’aide de nanofluides a` base de me´thanol au Al2O3 et au SiO2 dans un absorbeur a` colonne a` plateaux Mots cle´s : Absorption ; Ame´lioration ; Bulles ; Dioxyde de carbone ; E´coulement continu ; Me´thanol e Particule
1.
Introduction
Synthetic natural gas (SNG) is used in an integrated gasification combined cycle (IGCC) as fuel for combustion with the final purpose of producing energy. In an IGCC plant, acid gases such
as carbon dioxide (CO2) and hydrogen sulphide (H2S) need to be removed from the valuable feed gas streams. The acid gas removal (AGR) system produces a more suitable feed gas for combustion and further processing. This energy production technology is coming to the spot line as a consequence of the
* Corresponding author. Tel.: þ82 31 201 2990; fax: þ82 31 202 3260. E-mail address: [email protected] (Y.T. Kang). 1 IIR B1 Session, President. 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.03.017
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
Standard deviation, g s1
Nomenclature
s
B _ m
Subscripts abs absorption bf base fluid eff effective inlet inlet of test section nf nanofluid outlet outlet of test section
N R S t U w
Systematic uncertainty, % Mass flow rate, g s1 Number of measurements Absorption ratio Random uncertainty, % Absorption time, s Overall uncertainty, % Uncertainty, g s1
worldwide increase in the prices of crude oil and petrochemicals due to the oil scarcity in present days. Therefore, much attention is being paid to different alternatives such as SNG, which is becoming one of the promising options for energy production. The Rectisol process, developed by Lurgi GmbH, is a physical absorption process widely used to remove the acid gases in the IGCC. In the Rectisol process, the methanol absorbent should be cooled down to a temperature of about 40 C at normal operational conditions. The need to refrigerate the solvent results in a high capital and operating cost and is one of the main disadvantages of this process (Korens et al., 2002). However, methanol offers the highest CO2 absorption capacity relative to other solvents (Esteban et al., 2000). Therefore, the purpose of this study is to enhance the CO2 absorption rate of methanol with the use of Al2O3 and SiO2 nanoparticles. Recent studies have shown an improvement in CO2 absorption by using nanofluids. Kim et al. (2008) developed silica nanofluids in water for application in CO2 absorption; the results showed a 79% of increase in CO2 absorption rate and a 24% increase in total absorption in a bubble type absorber. In addition, experiments in a bubble type absorber were carried out by Lee et al. (2011) with methanol-based fluids with alumina and silica particles at different concentrations. It was
1403
concluded that the maximum CO2 absorption enhancements compared to the pure methanol were 4.5% at 0.01 vol% of Al2O3 and 5.6% at 0.01 vol% of SiO2, respectively. In the present study, suspensions of Al2O3 and SiO2 nanoparticles in methanol (nanofluid) are produced and analyzed for the application of CO2 absorption in a tray column absorber. The objectives of this study are to find an optimum concentration of nanoparticles, and to determine the absorption performance enhancement by using the methanol-based nanofluids in a tray column absorber for the given geometrical and operational conditions, which has a completely different absorption mechanism from the bubble type absorber. A model for the absorption enhancement is also proposed, the bubble breaking model, which is based on the absorption experiments and the bubble visualization.
2.
Experiments
2.1. Experimental apparatus for the absorption experiment Fig. 1 shows the schematic diagram of the experimental apparatus. The continuous removal system is an acrylic tray
Fig. 1 e Schematic of the experimental apparatus.
1404
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 5 ( 2 0 1 2 ) 1 4 0 2 e1 4 0 9
2.3.
Table 1 e Geometrical conditions. Number of plates Total length [cm] Diameter Dc [cm] Tray spacing [cm] Area [cm2] Net area [cm2] Active area [cm2] Hole area [cm2] Vol. per plate [cm3] Hole diameter [cm]
12 78 3 5 7.07 6.22 5.37 0.54 3.11 0.1
column with twelve plates. The column is a sieve tray type and has flat perforated plates where the vapor velocity keeps the liquid from flowing down through the holes. Table 1 shows the geometric conditions of the tray column. The column is equipped with an oval gear flow meter (accuracy 1% of reading) to measure the flow rate of the absorbent. Two gas flow meters with the same characteristics measure the flow of CO2 gas at the inlet and outlet (accuracy 1% of full scale). A desiccant unit is located before the outlet CO2 flow meter to remove methanol from the CO2 flow. Four thermocouples (accuracy 1.5 C) are located at the outlet and inlet of the gas and liquid to measure the temperatures of both the gas and the liquid.
2.2.
Preparation of the nanofluids
Inlet conditions are listed in Table 2. The fluid flow is increased slowly until the desired mass flow rate value is reached. The absorbent enters the tray column and flows downwards within the test section. The CO2 gas is measured by a mass flow meter; it enters from the bottom and goes upward as shown in Fig. 1. In each plate, the CO2 gas keeps the liquid from flowing down through the holes, then the gas passes through the perforations and the bubbles form a froth, disengages from the froth and passes on to the next tray above as shown in Fig. 2. During the time the liquid and the gas are in contact, interphase diffusion occurs and the methanol with the absorbed CO2 flows out of the system from the bottom of the tower into a tank. On the other side, the remaining CO2 gas flows out of the system from the top of the tower and passes through the outlet mass flow meter to be measured.
2.4.
Data reduction
Data is transferred from the data acquisition unit to the dataflow program (VEE, Agilent Technologies) every 2 s for a total of 400 s. However, it takes some time to stabilize the flow rate of the liquid and the gas, therefore, we take 100 s of stable behavior for our calculations. The CO2 absorption rate and the effective absorption ratio which is defined as the ratio of the absorption rate of the nanofluids to that of the pure methanol are obtained by Eqs. (1) and (2), respectively. Z
The nanofluid absorbents are prepared with aluminum oxide (Al2O3 99.5%, 40e50 nm, Alfa Aesar) and silicon dioxide (SiO2 99.5%, 10e20 nm, SigmaeAldrich) nanoparticles suspended in methanol (base fluid). The particles are stabilized into the methanol using an ultra-sonicator. With the application of sonication, there is no need to use surfactants to obtain a good dispersion of the nanoparticles in methanol. The solution is continuously circulated and sonicated for 1 h inside a flow cell reactor.
_ abs ¼ m
Reff ¼
Table 2 e Operational inlet conditions for the tray column absorber. CO2 Purity [%] Inlet flux [kg s1] Inlet Pressure [kPa]
99.999 6.66 105 104 10
Methanol Purity [%] Inlet flux [kg s1] Temperature [ C]
99.8 0.145 22 1
Al2O3 Size [nm] Concentration [vol%]
40e50 0.005, 0.01, 0.05, 0.1
_ inlet dt m
Z
_ outlet dt m
Dt
_ abs;nf m _ abs;bf m
(1)
(2)
The physical meaning of the effective absorption ratio is the effectiveness of the nanofluids for CO2 absorption enhancement.
2.5.
SiO2 Size [nm] Concentration [vol%]
Procedures for the absorption experiment
Experimental error analysis
The experimental error analysis was performed based on Holman (2001). The experimental uncertainty is determined by calculating the uncertainty in the results based on the systematic uncertainty and the random standard uncertainty. The random uncertainty is calculated based on the standard deviation of the mean (SDOM). The experimental uncertainties ranged 1.6%e10.2% as shown in Table 3. Uε ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 B2ε þ S2ε
where, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ outlet $woutlet Þ2 _ inlet $winlet Þ2 þðm ðm Bε ¼ _ abs m
(3)
(4)
and, 10e20 0.005, 0.01, 0.05, 0.1
2sabs Sε ¼ pffiffiffiffi _ abs N$m
(5)
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Fig. 2 e Pictures of the absorption process in a tray for the different concentrations of Al2O3 in methanol.
Table 3 e Experimental error analysis of the experimental results. Concentration [vol%]
Measured CO2 flow [g s1 102]
CO2 flow uncertainty [g s1 103]
Uncertainty [%]
_ abs m
winlet
woutlet
Bε
Sε
Uε
Pure methanol
2.45
4.60
2.76
1.2
1.7
3.0
SiO2 0.005 0.01 0.05 0.1
2.58 2.67 2.68 2.59
4.57 4.57 4.55 4.54
2.63 2.56 2.53 2.60
1.1 1.1 1.1 1.1
2.7 0.9 0.4 1.1
4.2 2.0 1.6 2.3
Al2O3 0.005 0.01 0.05 0.1
2.45 2.53 2.68 2.53
4.56 4.55 4.56 4.56
2.71 2.65 2.55 2.66
1.2 1.1 1.1 1.1
3.2 7.1 2.9 2.0
4.8 10.2 4.4 3.3
3.
Results and discussion
3.1.
Dispersion stabilization and particle size
Fig. 3 shows the prepared nanofluids at different Al2O3 and SiO2 concentrations right after the preparation and 24 h after the preparation. From the visual observations, we can see no
Fig. 3 e Pictures of the prepared nanofluids at different concentrations of SiO2 and Al2O3 right after the preparation and 24 h after.
Fig. 4 e Particle size of the absorbents at different SiO2 concentrations after 24 h.
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Fig. 8 e Bubble breaking model (The nanoparticles cover the bubble and break them into smaller bubbles). Fig. 5 e Particle size of the absorbents at different Al2O3 concentrations after 24 h. Table 4 e Enhancements obtained for the different concentrations of Al2O3 and SiO2. Concentration [vol%]
Fig. 6 e Absorption rate of the nanofluids prepared with Al2O3.
Fig. 7 e Absorption rate of the nanofluids prepared with SiO2.
Enhancement [%]
Al2O3 0.005 0.01 0.05 0.1
0.2 3.4 9.4 3.5
SiO2 0.005 0.01 0.05 0.1
5.6 9.0 9.7 5.9
clear sign of sedimentation for the concentrations of 0.005 and 0.01 vol% for both particles. To verify the stabilization of the prepared nanofluids, the size of the nanoparticles were measured by dynamic light scattering (DLS). Figs. 4 and 5 show the particle sizes at different concentrations for the absorbents prepared with Al2O3 and SiO2 respectively. The
Fig. 9 e Effective absorption ratio at different concentrations of Al2O3 and SiO2.
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absorbents with concentrations from 0.005 to 0.1 vol% show similar values after 24 h, confirming the good stability of the solutions. However, it is known that for higher concentrations of both Al2O3 and SiO2 (e.g. 0.5vol%), the particle size measurements after 24 h show a smaller particle size (Lee
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et al., 2011), which is a sign of sedimentation and could be caused by the clustering of the particles. It is believed that too much clustering may lead to large regions of “article free” liquid which in the case of heat transfer may exert a negative effect (Keblinski et al., 2002). Therefore, it is concluded that
Fig. 10 e Pictures of CO2 bubbles (500 fps) in methanol and nanofluids at different concentrations of Al2O3 and SiO2. (*) indicates the detached bubbles.
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the concentration ranges between 0.005 and 0.1 vol% of Al2O3 and SiO2 nanoparticles are recommended for good dispersion stability of the methanol-based nanofluids.
3.2.
Absorption rate
The absorption rate, in grams per second, is shown in Fig. 6 for Al2O3 and Fig. 7 for SiO2, respectively. It is found that the absorption rates enhance up to 9.4% for Al2O3 and 9.7% for SiO2 at the critical concentration of 0.05 vol%, after which the absorption rate decreases. Therefore, the critical concentration of 0.05 vol% is considered as an optimum concentration for CO2 absorption enhancement in methanol-based nanofluids with Al2O3 and SiO2 nanoparticles. The mechanism that explains such mass transfer enhancement is still speculative. Some theories such as the hydrodynamic effect propose that the particles may increase the specific interfacial area by covering the bubble surface and preventing the coalescence of the bubbles, resulting in smaller bubbles (Linek et al., 2008). Another possibility is that the particles may collide inducing local turbulence and refreshing the gas-liquid boundary layer by mixing it into the bulk liquid (Ruthiya et al., 2006). However, the general mechanism of the mass transfer enhancement is still unknown. In the work by Krishnamurthy et al. (2006), it is concluded that the Brownian motion of the nanoparticles is not directly responsible for the mass transfer enhancement, but mainly due to the velocity disturbance field in the fluid created by the motion of the nanoparticles. Most of the mass transfer enhancement mechanisms are developed for bubble systems (bubble columns, stirred tanks). However, in a dynamic flow system such as a tray column absorber, the forces induced by the movement of the liquid and vapor are believed to play more dominant roles in the mass transfer enhancement. In the bubble breaking model as shown in Fig. 8, the nanoparticles suspended in the base fluid cover the bubble, and as the movement of the fluid due to external forces become more dynamic, the particles collide with the gas-liquid interface, breaking the bubble into smaller size bubbles. More bubbles mean a larger interfacial area which would promote the mass transfer from the gas to the liquid. Table 4 shows the results of the enhancements achieved by the use of methanol-based nanofluids with SiO2 and Al2O3 nanoparticles. From Table 4, it is concluded that SiO2 nanoparticle is a better candidate for CO2 absorption enhancement than Al2O3 nanoparticles. Fig. 9 shows the effective absorption ratio, which is defined in Eq. (2). The effective absorption ratio increases as the particle concentration increases and both nanoparticles follow a similar trend. After reaching its critical point at 0.05 vol%, both absorbents reduce their enhancement effect and an opposite effect takes place. This behavior has been studied and it has been found that the self-diffusion coefficient decreases as the concentration of nanoparticles, in the liquid phase, increases. Gerardi et al. (2009) reported a decay trend in the diffusion coefficient for Al2O3/water and suggest two effects promoting this behavior: first, the tortuosity of the diffusion path of the water molecules is increased when solid particles stand in their way; second, the water molecules in the ordered layer on the surface of the particles are “bound” and move with the particles, which have a lower diffusion
coefficient than the free molecules. In summary, at low concentrations the movement of the particles promote mass transfer, however after the critical phase (an optimum concentration of 0.05 vol% in the present study), the nanoparticles become too dense in the liquid phase reducing the self-diffusion coefficient and consequently reducing the absorption of the gas phase.
3.3.
Bubble visualization
Additional visualization experiments were performed in order to observe the behavior of the CO2 bubbles in methanol/Al2O3 and methanol/SiO2 nanofluids. A Hele-Shaw cell with a separation of 5 mm between walls conforms the test section. The orifice is a rectangular shape of 5 7 mm. This configuration is applied to make possible the visualization of the nanofluids with the highest concentrations of 0.005 vol% and 0.1 vol%. A syringe pump is utilized to insert the gas into the liquid. Pictures are taken with a high-speed camera at 500 frames per second. From the pictures in Fig. 10, we can observe, in the pictures marked with (*), that the CO2 bubbles with higher concentration detach faster from the orifice. The detachment of the CO2 bubbles in pure methanol occurs at 68 ms while the detachment for SiO2 and Al2O3 nanofluid at 0.1 vol% occurs at 60 ms and 62 ms, respectively. The faster detachment induces a higher frequency rate which is consistent with the experimental work by Liang-Shih Fan et al. (2007). The higher frequency yields a smaller bubble size which promotes the mass transfer to the liquid phase. This result supports the CO2 absorption enhancement model proposed in the present study: the bubble breaking model.
4.
Conclusions
In this study, nanofluids made with Al2O3 and SiO2 nanoparticles at different concentrations in a methanol base fluid are produced and tested in a tray column absorber for CO2 absorption. The results are as follows: 1) It is concluded that the concentration ranges between 0.005 and 0.1 vol% of Al2O3 and SiO2 nanoparticles are recommended for good dispersion stability of the methanolbased nanofluids. 2) With the use of Al2O3 and SiO2 nanofluids in the tray column absorber, we can obtain a higher absorption rate compared with the pure methanol under the same conditions. It is found that the absorption rates of Al2O3 and SiO2 nanofluids enhance up to 9.4% and 9.7% respectively at the critical concentration of 0.05 vol%. It is also found that the SiO2 nanoparticle is a better candidate than Al2O3 nanoparticle and 0.05 vol% of nanoparticles is an optimum condition for CO2 absorption enhancement. 3) It is confirmed, from the visualization results, that the use of Al2O3 and SiO2 in methanol-based nanofluids promotes the detachment of the bubbles form the orifice, resulting in a higher frequency rate and therefore, enhancing the absorption rate.
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Acknowledgment This work was supported by the National Research Foundation (NRF) grant (No. 20100029120)
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