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Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field
Journal Pre-proof Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field
N. Abbaspour, M. Haghshenasfard, M.R. Talaei, H. Amini PII:
S0167-7322(19)36207-5
DOI:
https://doi.org/10.1016/j.molliq.2020.112689
Reference:
MOLLIQ 112689
To appear in:
Journal of Molecular Liquids
Received date:
9 November 2019
Revised date:
9 February 2020
Accepted date:
10 February 2020
Please cite this article as: N. Abbaspour, M. Haghshenasfard, M.R. Talaei, et al., Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field, Journal of Molecular Liquids(2020), https://doi.org/ 10.1016/j.molliq.2020.112689
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© 2020 Published by Elsevier.
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Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field N. Abbaspour1, M. Haghshenasfard*1, M.R. Talaei2, H. Amini1 1
Department of Chemical Engineering, Isfahan University of Technology, Isfahan, 8415683111, Iran Chemical Engineering Department, Shiraz University, Shiraz, Iran
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*Corresponding author: E-mail address: haghshenas@ iut.ac.ir (M. Haghshenasfard)
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Fax: +983133912677, Tel: +989131652500
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Abstract
In this study, the effects of Fe3O4/water nanofluid on the CO2 removal efficiency in a venturi
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scrubber equipped with a magnetic field were investigated. The results demonstrated the positive effects of both the magnetic field and magnetic nanoparticles on the CO2 removal efficiency. The
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optimum conditions were determined by using the Taguchi method. In the presence of the magnetic field and with the use of magnetic nanoparticles under the optimum terms, CO2
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removal percentage reached 17.56%, while it was 8.61% for the water without the magnetic
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field. On the other hand, the pressure drop in the venturi scrubber was increased in the presence of the magnetic field and increased by increasing the nanofluid concentration. Keywords: venturi scrubber, magnetic nanoparticles, magnetic field, CO2 Absorption
1. Introduction Increasing global warming by more than 2 degrees has become one of the most critical environmental challenges. CO2, as the most effective gas in global warming, with the share of 82.5% among greenhouse gases, is emitted as much as 30×10-12 kg in the atmosphere annually. Due to the environmental impact of the CO2 enhancement, its removal and reduction methods
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Journal Pre-proof have been the focus of attention [1-2]. One of the most efficient gas removal devices is venturi scrubbers. Venturi scrubbers can remove gases and solid pollutants from the contaminated gas streams in a single-stage process [3-5]. According to the high relative velocity between the gas and liquid droplets in a venturi scrubber, the mass transfer rate is significant, and therefore the venturi scrubbers are very useful to remove the gaseous pollutants such as CO2 from a gas stream. The performance of a venturi scrubber depends on a large number of factors, including the droplet dispersion, droplet diffusion, pressure drop, droplet atomization, and droplet size. The liquid injection method, which significantly predicts these essential factors to optimize a process
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in the venturi scrubber, is also preferred.
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A venturi scrubber consists of three main parts, a converging section, a throat section, and a diverging section. The gas-phase enters the converging part, and the gas velocity and turbulence
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rate increases. The liquid phase also comes from the entrance to the converging section or the
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throat section. In the converging part, the gas accelerates, causing the liquid phase to atomize and
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turn into tiny droplets.
In the design and optimization of a venturi scrubber, it is necessary to know the operating characteristics such as the droplet distribution, pressure drop, and mass transfer between the
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liquid and gas across the venturi scrubber. Some investigators have attempted to study the effect of these operating characteristics on the performance of the venturi scrubbers.
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Lehner [6] showed that by increasing the gas and liquid flowrates, the total pressure drop
particles.
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increases in a venturi scrubber utilizing water as a scrubber of polluted gas with titanium dioxide
Silva et al. [7] studied the effects of gas velocity and liquid flow rate on the pressure drop in a large-scaled venturi scrubber. Two different methods for liquid injection were examined. The observations showed that the effect of gas velocity on the total pressure drop is higher than that of the liquid flow rate. Furthermore, they found that the L/G ratio is more effective than the gas velocity in the throat section. There is a range of the L/G ratio at which the efficiency reaches the maximum amount. Besides, increasing the liquid load does not improve efficiency. Viswanathan et al. [8] investigated the effects of changing the L/G ratio and gas velocity on the droplets distribution and liquid film on the venturi walls. They showed that the droplet 2
Journal Pre-proof distribution highly depends on the L/G ratio, and there is a range of L/G ratios (7
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the pressure drop in venturi scrubbers. For increasing the efficiency of neural networks, genetic algorithm was used to optimize the parameters of the neural network [11]. The results showed
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that the model of neural network optimized by genetic algorithm was the best model due to its agreement with experimental data and greater flexibility compared to the mathematical models.
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Johnston et al. conducted one of the first studies about pollutant gas removal through the
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utilization of the venturi scrubber [12]. In this study, the absorption of SO2 by alkaline solutions (NaOH) and variation of the mass transfer coefficients with distance from the injection point
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were investigated. The results indicated that the venturi scrubbers are the new interesting contacting device for removing gaseous pollutants.
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Gamisans et al. [13] examined the SO2 absorption by NaOH solution and studied the effect of change in the throat length and diameter on the gas absorption. Likewise, two types of sprays
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were examined and the experimental values were compared with the predicted results of a model
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based on the rapid reaction in the liquid phase. The results indicated that the liquid film plays an essential role in the mass transfer rate. Talaei et al. [14] presented a three-dimensional mathematical model to predict the removal efficiency in a venturi scrubber for SO2 absorption with alkaline solutions. The results of the model were compared with the experimental data and good agreement was observed. Taheri et al. [15] developed a three-dimensional mathematical model, based on annular twophase flow, to simulate the gas absorption process in a venturi scrubber. In their work, the effects of droplet concentration distribution on the removal efficiency are studied. The results of the simulation are compared with the experimental data and mathematical models and showed an improvement over Talaei’s model [14]. 3
Journal Pre-proof One of the ways to increase the mass transfer rate between gas and liquid phases is to use the nanofluids [16-17]. The nanofluid is created by stabilizing nanoparticles with a diameter below 100 nanometers in a base fluid. Ferro-fluids or magneto-hydrodynamic nanofluids are a colloidal mixture of magnetic nanoparticles such as Fe3O4 in a base fluid [18]. Nowadays, the use of nanofluid in various industries due to its unique properties of enhancing mass and heat transfer has attracted significant attention from the researchers [19-21]. The mass transfer characteristics of magnetic nanofluids can be changed under the external magnetic field. This process is limited to a little number of published works in the conventional mass transfer devices such as wetted
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wall or packed column [22-24].
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Komati and Suresh [22] investigated CO2 absorption in amine /Fe3O4 magnetic nanofluids in a wetted-wall column. The results showed a rise in the absorption rates by the enhancement of the
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nanofluid volume fraction.
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Wu et al. [23] studied the effect of magnetic nanofluid on absorption performance in a bubble column equipped with an external magnetic field. The obtained results revealed that by
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increasing the nanofluid volume fraction and magnetic field intensity, the removal rate of ammonia increases.
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Samadi et al. [24] studied the effect of Al2O3, Fe3O4, and TiO2 nanoparticles and an external magnetic field on the CO2 absorption in a wetted wall column. Their observations showed that in
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a volume fraction of 1% vol. of Al2O3/ water nanofluid, the mass transfer rate was increased by 40-55%. The application of the magnetic field increased the mass transfer flux and the mass
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transfer coefficient up to 59% and 22.35%, respectively. The primary purpose of the present work is to investigate the effect of magnetic nanoparticles on the CO2 absorption, pressure drop, and droplet distribution in a venturi scrubber equipped with an external magnetic field. The optimum conditions are determined by using the Taguchi method, and the effects of the magnetic field intensity and nanoparticles concentration on the CO2 removal efficiency and pressure drop are investigated.
2. Experimental set-up and procedures Fig. 1 illustrates the schematic of the experimental apparatus. The apparatus consists of a pilotscale venturi scrubber equipped with an external magnetic field, CO2 cylinder, cyclone, sampling 4
Journal Pre-proof system, air blower, manometer, gas flowmeter, pump, and nanofluid container. The venturi scrubber with a cylindrical cross-section includes convergence, throat, and divergence parts with a length of 46, 20, and 36 cm, respectively. The diameter of the entrance region and throat are 16 and 8cm, respectively. The nanofluid was injected after adjustment by a flowmeter at the centerline of the throat inlet axially, and the CO2 was entered with air stream from the top of the
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convergence area.
Fig 1 -The schematic diagram of experimental set-up
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In this study, Fe3O4 nanoparticles with a particle diameter of 30nm were used, and Fe3O4/water nanofluid with volume fractions of 0.001, 0.01, and 0.02 were made. To prevent the deposition and aggregation of nanoparticles, the nanofluid was stirred using mechanical agitator and highpower ultrasonic vibrator for 4 hr.
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A U-type manometer has been used to measuring the pressure drop. The pressure drop across the venturi was measured at four points, shown in Fig. 1 (p1, p2, p3, and p4). As shown in Table 1, the
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pressure drops under different gas and liquid flowrates were measured for water and 0.01%
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nanofluid.
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Table1- The operating conditions in the pressure drop experiments Parameters
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Liquid Flow Rate (L/min)
Injected Liquid
1 25
Pure Water
2 30 40 50 0.01 V/V% Nanofluid
With /Without magnetic Field
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Magnetic Field
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Air Velocities(m/s)
Levels
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A sampling system includes a sampling tube, small-scale cyclone, a valve, and a vacuum pump that has been used for measuring the droplet distribution across the throat. An isokinetic sampling technique was used for the liquid droplet measurements. Under the isokinetic conditions, the gas flow rate at the entrance of the sampling tube is equal to the gas flowrate through the system. The sampling tube had an inner diameter of 3mm. The small cyclone removed the droplets entrained in the sampler flow. By changing the position of the narrow tube radially across the throat, accumulated droplets at each point can be determined. The liquid samples were taken across the cross section of the throat outlet in eight radial positions. Using the weight of the liquid collected in the cyclonic separator, local liquid flux values can be calculated.
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Journal Pre-proof For calculation of the CO2 absorption rate, the collected samples were quickly prepared to titration and pH measurement. The concentration of absorbed CO2 in the liquid phase was determined using titration with sodium hydroxide and hydrochloric acid. As shown in Table 2, the effects of nanofluid concentration, air velocity, liquid flowrate, and CO2 flow rate on the removal rate of CO2 are studied. All experiments were carried out under constant temperature. Table2- The operating conditions in the absorption experiments Parameters
Levels 0
0.001
0.01
0.02
Air Velocity m/s
25
30
40
50
Liquid Flow Rate L/min
0.25
0.5
1
1.5
CO2 Flow Rate L/min
2
3
4
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Nanofluid Concentration V/V
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In this section, the Taguchi method was utilized to consider the optimal conditions with a
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minimum number of tests. The system studied consisted of four parameters, including the liquid flow rate, air velocity, nanoparticle volume fraction, and the CO2 flow rate, as presented in four
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levels in Table 3. Each experiment was repeated twice. A set consisting of 16 experiments was suggested by the Taguchi method and this was adjusted with the L16 orthogonal array. The
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results of the designed experiments could be investigated using the signal-to-noise ratio (S/N), which represents the quality of the parameter deviation from the desired values [25].
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In order to calculate the S/N ratio, the mean square of the deviations (MSD) was used. Regarding the feature of the system response, three types of MSD have been defined: MSD=
2
∑
MSD= ∑
2
MSD= ∑
Smaller is better Larger is better Nominal value is better
n= Number of experiments Y. = initial experimental data 7
Journal Pre-proof Yi= Experimental data Considering the output characteristic understudy, which was the CO2 removal efficiency, the equation of "bigger is better" was selected to calculate the S/N ratio. Then, the following relation was used to calculate the S/N ratio: S/N=-10 log (MSD) Table3- Experiments presented by the Taguchi method
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25 25 25 25 30 30 30 30 40 40 40 40 50 50 50 50
Nanofluid volume fraction vol%
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Liquid flowrate (L/min) 0.25 0.5 1 1.5 0.25 0.5 1 1.5 0.25 0.5 1 1.5 0.25 0.5 1 1.5
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Air velocity (m/s)
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Main factors
0.0 0.001 0.01 0.02 0.001 0.0 0.02 0.01 0.01 0.02 0.0 0.001 0.02 0.02 0.001 0.0
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Exp. No
CO2 flowrate (L/min) 2 3 4 5 4 5 2 3 5 4 3 2 3 2 4 5
In order to produce the magnetic field, five single-ring magnets with the same strength has been used around the throat. Fig. 2 shows the intensity of the magnetic field, measured by a gauss meter, versus throat radius.
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Intensity of magnetic field (G)
1400 1200 1000 800 600 400 200
10 20 30 Throat radius (mm)
40
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0
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Fig. 2- magnetic field intensity versus throat radius
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3. Results and discussion 3.1 Droplet distribution
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A narrow sampling tube connected to the vacuum pump and a small cyclone has been used for measuring the droplet distribution in the throat. The sample of the liquid phase was taken across
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the cross-section of the throat outlet in 8 radial positions. The local flux of the droplets is
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calculated using Eq. 1-3.
(1)
(2) In this equation, na is the local flux (
), mtube is the mass of collected droplets (kg), ATube is
the cross section area of the sampling tube (m2), and t is collecting time (s). The average flux of droplets Na (
) can be calculated using equation 2. m0 is the mass flow rate of the injected
fluid (kg/s), and Aventuri is the venturi cross section at the point in which the sampling tube is located (m2). Therefore, the droplet distribution or the normalized flux (R) can be considered as follows: 9
Journal Pre-proof (3) Fig. 3 shows the radial distribution of normalized flux at the end of the throat. The gas throat velocity is 50 m/s and the liquid flow rate is 1 L/min. The effect of using nanofluid (water/Fe3O4 0.01%) and the magnetic field also is shown in this figure. The slope of each graph is a measure of the uniformity of the droplet distribution. The lower slope indicates a more uniform droplet distribution. 5 4
ro
y = -0.0805x + 3.4208
3
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y = -0.0594x + 3.0329 2
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1 0 10
20 Radial distance (mm)
na
0
30
Water-with magnetic field Nanofluid-with magnetic field Linear (Water-without magnetic field)
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Normalized flux R
Water-without magnetic field
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y = -0.1158x + 4.1529
Linear (Water-with magnetic field) 40
Linear (Nanofluid-with magnetic field)
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Fig. 3- Radial variation of the normalized flux for Vg = 50 m/s and Q = 1 L/min
It is evident that the magnetic field is active on the distribution of the droplets of both water and nanofluid, and the use of a magnetic field increases the uniformity of the droplets distribution. Due to the magnetic field, droplets are distributed more uniformly across the throat as the slope decreases. Without the magnetic field, the liquid droplets tend to pass through the center region of the throat. As the magnetic field is applied, due to the bipolarity of the water droplets and the magnetic properties of the nanofluid, the droplets are diverted to the tube walls by the magnetic field (which is greater near the walls as shown in Fig. 2). The droplet distribution in water / Fe3O4 nanofluid is improved, due to the greater effectiveness of the magnetic nanoparticles in contact with the magnetic field.
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Journal Pre-proof Another factor, which has a remarkable influence on the distribution of the droplets, is the smaller diameter of the nanofluid droplets. According to the lower surface tension of water / Fe3O4 nanofluid compared to base fluid, smaller droplets are formed and more uniformity of droplets is observed [26].
3.2 Pressure drop
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The pressure drop through the throat was measured using a u-type accurate manometer. Fig. 4 shows the variations of the pressure drop in the throat versus air velocity. Pressure drop under
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different gas velocities with and without liquid injection for water and water/Fe3O4 nanofluid is shown. The liquid flowrates are 1 and 2 L/min in Fig. 4 (a) and (b), respectively and the volume
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fraction of Ferro-nanofluid is 0.01%. As shown in Fig. 4, by increasing the gas velocity, the dry
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pressure drop increases, due to the change in the gas momentum and the friction between the gas and the wall of the venturi's throat. The average dry pressure drop across the throat is about
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25.8% lower than the wet pressure drop (Q=1L/min). This trend has also been reported in other studies, such as Allen and Santen [27].
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It is clear that the pressure drop also depends on the liquid flow rate, as shown in Fig. 4 (a) and (b). A higher liquid flow rate means that more energy is needed to accelerate the droplets of the
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liquid; therefore by increasing the liquid flow rate, the pressure drop increases. Virkar and Sharma [28] reported the same results.
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The effect of the nanofluid and the magnetic field also are shown in Fig. 4. It is clear that the pressure drop of nanofluid is higher than the base fluid, distilled water.
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Pressure drop (pa)
200
(a)
150
100
50
0 25
30
35
40
Air velocity (m/s)
50
55
Nanofluid without magnetic field
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Nanofluid with magnetich field
45
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20
Water with magnetic field
Water without magnetic field
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Dry pressure drop
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(b)
200
na
150
50 0 20
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100
25
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Pressure drop (pa)
250
30
35
40
45
50
55
Air velocity (m/s)
Nanofluid with magnetich field
Nanofluid without magnetic field
Water with magnetic field
Water without magnetic field
Dry pressure drop
Fig. 4- Effect of gas velocity and magnetic field on dry and wet pressure drop (a) Q=1L/min, (b) Q=2L/min As the Fe3O4/water nanofluid concentration increases, the density and viscosity increases, while the surface tension decreases [29]. Consequently, with reducing the surface tension, smaller droplets were formed, causing the droplets to get more kinetic energy when accompanied by 12
Journal Pre-proof more pressure drop of gas. For instance, according to Fig. 4 (b), the pressure drop of nanofluid under the constant velocity of Vg= 30 m/s is 33%% higher than that for distilled water. Regarding the influence of the magnetic field on the pressure drop, it is clear that using the magnetic field in both distilled water and nanofluid led to an increase in the pressure drop. For instance, according to Fig. 4 (b), in a system with the magnetic field and Vg=40 m/s, the enhancement in pressure drop in water and nanofluid is 50% and 64%, respectively. The reason for this enhancement can be the diameter contraction of the liquid droplets in the presence of the
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magnetic field, as described in the previous section.
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Fig. 5 (a) and (b) shows the pressure loss within the converging, throat, and diverging sections. The effects of gas velocity, nanofluid, and magnetic field are shown in this figure and the trends
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are the same as Fig. 4.
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As shown in Fig. 5 (a) and (b), in the converging section, most of the pressure drop is due to the acceleration of the gas. Friction also increases the pressure drop in the throat. In the divergence
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both distilled water and nanofluid.
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section, the pressure drop is partially recovered by up to 70% [30]. This behavior is the same for
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(a) 1400
pressure drop (pa)
1200 with magnetic field V=30m/s
1000 800 600
without magnetic field V=30m/s
400
with magnetic field V=50 m/s
200
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without magnetic field V=50 m/s
0 0
20
40
60
80
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distance (cm)
100
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(b) 1400
lP
1000
without magnetic field V=30m/s
na
800
with magnetic filed V=30 m/s
600 400
with magnetic field V=50m/s
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pressure drop (pa)
1200
0 0
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200 20
40
without magnetic field V=50 m/s 60
80
100
distance (cm)
Fig. 5- Pressure variation for water and nanofluid within venturi scrubber (a) distilled water (b) water/Fe3O4 nanofluids 0.01% V/V, Q=1L/min
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Journal Pre-proof 3.3 Absorption of CO2 3.3.1 Removal efficiency without magnetic field Fig. 6 shows the average S/N ratio and CO2 removal percentage versus four effective factors. The effect of gas velocity, liquid flowrate, nanofluid concentration, and CO2 flowrate on the CO2 removal efficiency is shown. The removal percentage can be calculated using the following relation:
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(4)
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Which, Cin is CO2 concentration at venturi inlet (mol/L) and C0 is CO2 concentration at the
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na
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venturi outlet (mol/L).
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Fig 6-Effects of important factors on the CO2 removal efficiency and average S/N ratio (without magnetic field) It is clear that the enhancement of all four parameters has a positive effect on the CO2 absorption. These observations were consistent with other studies such as Gamisans et al. [10]. By enhancing 16
Journal Pre-proof the gas velocity, the waves formed will be stronger and higher at the surface of the liquid jet, which increases the droplet break up rate. Hence, spray formation began with increasing the gas velocity in a shorter distance. As the gas velocity in the throat increases, the liquid droplets break into the smaller droplets, thus, the contact surface between the phases enhances; thereby, the mass transfer rate increases [31-34]. By increasing the CO2 flowrate, in fact, the driving force is increased, and therefore the CO2 removal efficiency is improved. Regarding Fig. 6, the CO2 removal percentage is enhanced by
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% concentration is 25.2% higher than that in distilled water.
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increasing the nanoparticles volume fraction. CO2 removal efficiency in the nanofluid with 0.02
Brownian motion of nanoparticles and grazing effect are two important mechanisms, which
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increase the mass transfer rate of nanofluids. By adding nanoparticles in the base fluid, the micro convection caused by the Brownian motions of the nanoparticles increases the mass transfer rate.
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The grazing effect is related to the gas adsorption in the layer around the nanoparticles in the
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gas-liquid interface [1].
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3.3.2 Removal efficiency with the magnetic field In this section, the CO2 removal efficiency in the venturi scrubber equipped with a non-uniform
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magnetic field is investigated. The experiments were carried out based on four factors of throat
3.
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gas velocity, liquid flow rate, nanoparticle volume fraction, and CO2 flow rate, according to table
The results are shown in Fig. 7. By comparing Figs. 6 and 7, it is evident that using the magnetic field in all experiments improved the removal percentage. By placing a Ferro-fluid (Fe3O4/water nanofluid) alongside the magnetic field, a regular pattern of peaks-valleys is created on the surface of the nanofluid. This effect, which is known as the Rosensweig or normal-field instability, will make the surface wavy, therefore the surface free energy and turbulence of the liquid increases [35]. The turbulence of the liquid flow improves the mass transfer performance. By increasing the gas and liquid flowrate, the removal efficiency increases. It can be found that the removal efficiency of nanofluid is higher than that in the base fluid. However, the effect of nanofluid concentration on the removal efficiency under the magnetic field differs from that 17
Journal Pre-proof without the magnetic field. By increasing the nanofluid concentration to an optimal point, the removal efficiency increases, and then decreases. The optimum volume fraction of magnetic nanofluid is 0.001 %, which the enhancement of removal efficiency in comparison with distilled water is 33%. As mentioned earlier, this enhancement is due to the grazing effect and Brownian motion. However, the reduction of removal rate in high concentrations is might be due to nanoparticles agglomeration, nanoparticle settling and nanofluid instability in the presence of the magnetic field. Philip et al. [36] obtained similar results by considering Fe3O4/water nanofluid. In a magnetic nanofluid, each magnetite nanoparticle is a single domain superparamagnetic with
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a magnetic moment, and the magnetic nanoparticles form chainlike structure. Under absent of
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the magnetic field, the magnetic moments are oriented in random direction. By increasing the magnetic intensity, the moments of the nanoparticles start to align themselves along the direction
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of the magnetic field. Increasing the nanofluid volume fraction leads to the formation of chains extending to the entire volume of the sample cell. The reduction of mass transfer in high
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concentrations is expected to be due to the cross-linking of the chains and distortion in the
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nematic-like order.
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Fig. 7- Effects of important factors on the CO2 removal efficiency and average S/N ratio (with magnetic field) The analysis of variance, ANOVA, is presented in Table 4. ANOVA is used to determine the relative importance of various factors on the experiment response (CO2 removal efficiency). In ANOVA analysis, the values of freedom degree (DOF), sum of squares, variance, F-Value, P19
Journal Pre-proof value, and percent of contribution (P, %) for each factors in response are obtained. It can be observed that the gas velocity in the throat has the most contribution (36.89%) among the factors in the CO2 removal efficiency and the error percent in the response is very low (3.06%). Given the values, the degree of freedom for each factor is three. According to the data reported in table 4, all of the factors considered efficacious; besides, the error value is less than 4%. Table 4- Analysis of variance for CO2 removal efficiency DOF
Adj SS
Adj MS
F-Value
P-value
P(%)
Gas Throat
3
18.202
54.607
12.05
0.035
36.89
Liquid Flow Rate
3
9.337
28.012
6.18
0.084
18.92
Nanofluid
3
10.916
32.747
7.23
0.069
22.12
CO2 Flow Rate
3
9.374
28.122
6.21
0.084
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Error
3
1.511
Total
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Velocities (m/s)
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Factors
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Concentration
3.06
na
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4.532
Further experiments were carried out for a closer look at the effect of the magnetic field on the
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CO2 absorption in magnetic nanofluids. The confirmation study after getting the optimized conditions also are conducted to verifying the experimental design results based on Taguchi
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method. According to the Taguchi data and analysis of variance, it was found that the optimum operating conditions for maximum carbon dioxide absorption in the nanofluid are Vg = 50m/s, Ql = 1.5 L/min, and QCO2 = 5 L/min. The effect of nanofluid volume fraction and magnetic field on the removal percentage of CO2 under the optimum conditions is shown in Fig. 8.
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Removal percentage %
20
Without Magnetic Field
15
With Magnetic Field 10
0
0.005
0.01
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5 0.015
0.02
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Nanofluid volume fraction
0.025
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Fig. 8- Effect of nanofluid volume fraction and magnetic field on the removal percentage of CO2
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(Vg=50 m/s,Ql=1.5 L/min,QCO2=5 L/min)
It is clear the magnetic field enhanced the removal percentage of CO2; besides, the absorption
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rate of CO2 in nanofluid is higher than that in the base fluid. For instance, in 0.02% nanofluid without magnetic field, the removal rate is 20% higher than that in the pure water; also, the
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average removal rate of nanofluid under the magnetic field is about 26.2% higher than removal rate without magnetic field.
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Another matter that is clear from this figure is that the optimum volume fraction of nanofluid is 0.001, which confirms the experimental design results presented in Fig.7. After this point, in the absence of a magnetic field, the increase in removal rate remains almost constant, and under the magnetic field, the removal rate decreases. The decrease in removal rate after volume fractions of 0.001 may be due to the deposition of nanoparticles and instability of dense nanofluid in the presence of the magnetic field.
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Journal Pre-proof 4. Conclusion The purpose of this work was to study the effect of Fe3O4/water nanofluid on CO2 removal efficiency in a venturi scrubber in the absence and presence of an external magnetic field. The absorption rate, pressure drop and droplet distribution were investigated. The results showed that the droplet distribution in Fe3O4/water nanofluid was more uniform in comparison with distilled water, and the use of the magnetic field increases the uniformity of the distribution of the droplets. Although the distribution of the droplets was improved using the magnetic field, but the
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pressure drop was increased significantly. Besides, the pressure drop increased by increasing the nanofluid concentration. The results showed that the optimum volume fraction of nanofluid was
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0.001 vol%, which compared to pure water, increased the removal efficiency in the absence and
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in the presence of the magnetic field by up to 19.6% and 36.3%, respectively.
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[28] P.D. Virkar, M.M. Sharma, Mass transfer in venturi scrubbers, Can. J. Chem. Eng. 53 (1975) 512–516. https://doi.org/10.1002/cjce.5450530509 [29] D. Chicea, C. M. Gonce, On aqueous Fe3O4 nanofluid room temperature synthesis and physical properties, Optoelectron. Adv. Mat., 3(2009) 185-189 [30] A.Silva, The hydrodynamics of two-phase annular flow in venturi. PhD thesis,Chemical Engineering department, University of Minho, (2009). [31] D.B. Roberts, J.C. Hill, Atomization in venturi scrubbers, J. Chem. Eng. Commun. 12 (1981) 33–68. https://doi.org/10.1080/00986448108910830 [32] T.D. Placek, .L.K. Peters, Analysis of particulate removal in venturi scrubbers-effect of operating
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Journal Pre-proof CRediT author statement Nastaran Abbaspour: Conceptualization, Software Masoud Haghshenasfard: Supervision, Writing- Original draft preparation, Methodology Mohammadreza Talaei: Conceptualization, Visualization, Investigation.
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Hasanali Amini: Resources
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Journal Pre-proof Highlights: Magnetic field is an effective parameter on CO2 absorption rate and pressure drop
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Droplet distribution in nanofluid is more uniform in comparison with pure water
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Use of magnetic field increases the uniformity of the droplets distribution
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The pressure drop increases by increasing the nanofluid concentration
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N. Abbaspour, M. Haghshenasfard, M.R. Talaei, H. Amini PII:
S0167-7322(19)36207-5
DOI:
https://doi.org/10.1016/j.molliq.2020.112689
Reference:
MOLLIQ 112689
To appear in:
Journal of Molecular Liquids
Received date:
9 November 2019
Revised date:
9 February 2020
Accepted date:
10 February 2020
Please cite this article as: N. Abbaspour, M. Haghshenasfard, M.R. Talaei, et al., Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field, Journal of Molecular Liquids(2020), https://doi.org/ 10.1016/j.molliq.2020.112689
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© 2020 Published by Elsevier.
Journal Pre-proof
Experimental investigation of using nanofluids in the gas absorption in a venturi scrubber equipped with a magnetic field N. Abbaspour1, M. Haghshenasfard*1, M.R. Talaei2, H. Amini1 1
Department of Chemical Engineering, Isfahan University of Technology, Isfahan, 8415683111, Iran Chemical Engineering Department, Shiraz University, Shiraz, Iran
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*Corresponding author: E-mail address: haghshenas@ iut.ac.ir (M. Haghshenasfard)
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Fax: +983133912677, Tel: +989131652500
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Abstract
In this study, the effects of Fe3O4/water nanofluid on the CO2 removal efficiency in a venturi
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scrubber equipped with a magnetic field were investigated. The results demonstrated the positive effects of both the magnetic field and magnetic nanoparticles on the CO2 removal efficiency. The
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optimum conditions were determined by using the Taguchi method. In the presence of the magnetic field and with the use of magnetic nanoparticles under the optimum terms, CO2
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removal percentage reached 17.56%, while it was 8.61% for the water without the magnetic
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field. On the other hand, the pressure drop in the venturi scrubber was increased in the presence of the magnetic field and increased by increasing the nanofluid concentration. Keywords: venturi scrubber, magnetic nanoparticles, magnetic field, CO2 Absorption
1. Introduction Increasing global warming by more than 2 degrees has become one of the most critical environmental challenges. CO2, as the most effective gas in global warming, with the share of 82.5% among greenhouse gases, is emitted as much as 30×10-12 kg in the atmosphere annually. Due to the environmental impact of the CO2 enhancement, its removal and reduction methods
1
Journal Pre-proof have been the focus of attention [1-2]. One of the most efficient gas removal devices is venturi scrubbers. Venturi scrubbers can remove gases and solid pollutants from the contaminated gas streams in a single-stage process [3-5]. According to the high relative velocity between the gas and liquid droplets in a venturi scrubber, the mass transfer rate is significant, and therefore the venturi scrubbers are very useful to remove the gaseous pollutants such as CO2 from a gas stream. The performance of a venturi scrubber depends on a large number of factors, including the droplet dispersion, droplet diffusion, pressure drop, droplet atomization, and droplet size. The liquid injection method, which significantly predicts these essential factors to optimize a process
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in the venturi scrubber, is also preferred.
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A venturi scrubber consists of three main parts, a converging section, a throat section, and a diverging section. The gas-phase enters the converging part, and the gas velocity and turbulence
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rate increases. The liquid phase also comes from the entrance to the converging section or the
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throat section. In the converging part, the gas accelerates, causing the liquid phase to atomize and
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turn into tiny droplets.
In the design and optimization of a venturi scrubber, it is necessary to know the operating characteristics such as the droplet distribution, pressure drop, and mass transfer between the
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liquid and gas across the venturi scrubber. Some investigators have attempted to study the effect of these operating characteristics on the performance of the venturi scrubbers.
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Lehner [6] showed that by increasing the gas and liquid flowrates, the total pressure drop
particles.
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increases in a venturi scrubber utilizing water as a scrubber of polluted gas with titanium dioxide
Silva et al. [7] studied the effects of gas velocity and liquid flow rate on the pressure drop in a large-scaled venturi scrubber. Two different methods for liquid injection were examined. The observations showed that the effect of gas velocity on the total pressure drop is higher than that of the liquid flow rate. Furthermore, they found that the L/G ratio is more effective than the gas velocity in the throat section. There is a range of the L/G ratio at which the efficiency reaches the maximum amount. Besides, increasing the liquid load does not improve efficiency. Viswanathan et al. [8] investigated the effects of changing the L/G ratio and gas velocity on the droplets distribution and liquid film on the venturi walls. They showed that the droplet 2
Journal Pre-proof distribution highly depends on the L/G ratio, and there is a range of L/G ratios (7
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the pressure drop in venturi scrubbers. For increasing the efficiency of neural networks, genetic algorithm was used to optimize the parameters of the neural network [11]. The results showed
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that the model of neural network optimized by genetic algorithm was the best model due to its agreement with experimental data and greater flexibility compared to the mathematical models.
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Johnston et al. conducted one of the first studies about pollutant gas removal through the
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utilization of the venturi scrubber [12]. In this study, the absorption of SO2 by alkaline solutions (NaOH) and variation of the mass transfer coefficients with distance from the injection point
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were investigated. The results indicated that the venturi scrubbers are the new interesting contacting device for removing gaseous pollutants.
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Gamisans et al. [13] examined the SO2 absorption by NaOH solution and studied the effect of change in the throat length and diameter on the gas absorption. Likewise, two types of sprays
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were examined and the experimental values were compared with the predicted results of a model
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based on the rapid reaction in the liquid phase. The results indicated that the liquid film plays an essential role in the mass transfer rate. Talaei et al. [14] presented a three-dimensional mathematical model to predict the removal efficiency in a venturi scrubber for SO2 absorption with alkaline solutions. The results of the model were compared with the experimental data and good agreement was observed. Taheri et al. [15] developed a three-dimensional mathematical model, based on annular twophase flow, to simulate the gas absorption process in a venturi scrubber. In their work, the effects of droplet concentration distribution on the removal efficiency are studied. The results of the simulation are compared with the experimental data and mathematical models and showed an improvement over Talaei’s model [14]. 3
Journal Pre-proof One of the ways to increase the mass transfer rate between gas and liquid phases is to use the nanofluids [16-17]. The nanofluid is created by stabilizing nanoparticles with a diameter below 100 nanometers in a base fluid. Ferro-fluids or magneto-hydrodynamic nanofluids are a colloidal mixture of magnetic nanoparticles such as Fe3O4 in a base fluid [18]. Nowadays, the use of nanofluid in various industries due to its unique properties of enhancing mass and heat transfer has attracted significant attention from the researchers [19-21]. The mass transfer characteristics of magnetic nanofluids can be changed under the external magnetic field. This process is limited to a little number of published works in the conventional mass transfer devices such as wetted
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wall or packed column [22-24].
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Komati and Suresh [22] investigated CO2 absorption in amine /Fe3O4 magnetic nanofluids in a wetted-wall column. The results showed a rise in the absorption rates by the enhancement of the
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nanofluid volume fraction.
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Wu et al. [23] studied the effect of magnetic nanofluid on absorption performance in a bubble column equipped with an external magnetic field. The obtained results revealed that by
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increasing the nanofluid volume fraction and magnetic field intensity, the removal rate of ammonia increases.
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Samadi et al. [24] studied the effect of Al2O3, Fe3O4, and TiO2 nanoparticles and an external magnetic field on the CO2 absorption in a wetted wall column. Their observations showed that in
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a volume fraction of 1% vol. of Al2O3/ water nanofluid, the mass transfer rate was increased by 40-55%. The application of the magnetic field increased the mass transfer flux and the mass
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transfer coefficient up to 59% and 22.35%, respectively. The primary purpose of the present work is to investigate the effect of magnetic nanoparticles on the CO2 absorption, pressure drop, and droplet distribution in a venturi scrubber equipped with an external magnetic field. The optimum conditions are determined by using the Taguchi method, and the effects of the magnetic field intensity and nanoparticles concentration on the CO2 removal efficiency and pressure drop are investigated.
2. Experimental set-up and procedures Fig. 1 illustrates the schematic of the experimental apparatus. The apparatus consists of a pilotscale venturi scrubber equipped with an external magnetic field, CO2 cylinder, cyclone, sampling 4
Journal Pre-proof system, air blower, manometer, gas flowmeter, pump, and nanofluid container. The venturi scrubber with a cylindrical cross-section includes convergence, throat, and divergence parts with a length of 46, 20, and 36 cm, respectively. The diameter of the entrance region and throat are 16 and 8cm, respectively. The nanofluid was injected after adjustment by a flowmeter at the centerline of the throat inlet axially, and the CO2 was entered with air stream from the top of the
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convergence area.
Fig 1 -The schematic diagram of experimental set-up
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In this study, Fe3O4 nanoparticles with a particle diameter of 30nm were used, and Fe3O4/water nanofluid with volume fractions of 0.001, 0.01, and 0.02 were made. To prevent the deposition and aggregation of nanoparticles, the nanofluid was stirred using mechanical agitator and highpower ultrasonic vibrator for 4 hr.
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A U-type manometer has been used to measuring the pressure drop. The pressure drop across the venturi was measured at four points, shown in Fig. 1 (p1, p2, p3, and p4). As shown in Table 1, the
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pressure drops under different gas and liquid flowrates were measured for water and 0.01%
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nanofluid.
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Table1- The operating conditions in the pressure drop experiments Parameters
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Liquid Flow Rate (L/min)
Injected Liquid
1 25
Pure Water
2 30 40 50 0.01 V/V% Nanofluid
With /Without magnetic Field
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Magnetic Field
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Air Velocities(m/s)
Levels
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A sampling system includes a sampling tube, small-scale cyclone, a valve, and a vacuum pump that has been used for measuring the droplet distribution across the throat. An isokinetic sampling technique was used for the liquid droplet measurements. Under the isokinetic conditions, the gas flow rate at the entrance of the sampling tube is equal to the gas flowrate through the system. The sampling tube had an inner diameter of 3mm. The small cyclone removed the droplets entrained in the sampler flow. By changing the position of the narrow tube radially across the throat, accumulated droplets at each point can be determined. The liquid samples were taken across the cross section of the throat outlet in eight radial positions. Using the weight of the liquid collected in the cyclonic separator, local liquid flux values can be calculated.
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Journal Pre-proof For calculation of the CO2 absorption rate, the collected samples were quickly prepared to titration and pH measurement. The concentration of absorbed CO2 in the liquid phase was determined using titration with sodium hydroxide and hydrochloric acid. As shown in Table 2, the effects of nanofluid concentration, air velocity, liquid flowrate, and CO2 flow rate on the removal rate of CO2 are studied. All experiments were carried out under constant temperature. Table2- The operating conditions in the absorption experiments Parameters
Levels 0
0.001
0.01
0.02
Air Velocity m/s
25
30
40
50
Liquid Flow Rate L/min
0.25
0.5
1
1.5
CO2 Flow Rate L/min
2
3
4
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Nanofluid Concentration V/V
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In this section, the Taguchi method was utilized to consider the optimal conditions with a
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minimum number of tests. The system studied consisted of four parameters, including the liquid flow rate, air velocity, nanoparticle volume fraction, and the CO2 flow rate, as presented in four
na
levels in Table 3. Each experiment was repeated twice. A set consisting of 16 experiments was suggested by the Taguchi method and this was adjusted with the L16 orthogonal array. The
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results of the designed experiments could be investigated using the signal-to-noise ratio (S/N), which represents the quality of the parameter deviation from the desired values [25].
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In order to calculate the S/N ratio, the mean square of the deviations (MSD) was used. Regarding the feature of the system response, three types of MSD have been defined: MSD=
2
∑
MSD= ∑
2
MSD= ∑
Smaller is better Larger is better Nominal value is better
n= Number of experiments Y. = initial experimental data 7
Journal Pre-proof Yi= Experimental data Considering the output characteristic understudy, which was the CO2 removal efficiency, the equation of "bigger is better" was selected to calculate the S/N ratio. Then, the following relation was used to calculate the S/N ratio: S/N=-10 log (MSD) Table3- Experiments presented by the Taguchi method
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na
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25 25 25 25 30 30 30 30 40 40 40 40 50 50 50 50
Nanofluid volume fraction vol%
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Liquid flowrate (L/min) 0.25 0.5 1 1.5 0.25 0.5 1 1.5 0.25 0.5 1 1.5 0.25 0.5 1 1.5
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Air velocity (m/s)
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Main factors
0.0 0.001 0.01 0.02 0.001 0.0 0.02 0.01 0.01 0.02 0.0 0.001 0.02 0.02 0.001 0.0
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Exp. No
CO2 flowrate (L/min) 2 3 4 5 4 5 2 3 5 4 3 2 3 2 4 5
In order to produce the magnetic field, five single-ring magnets with the same strength has been used around the throat. Fig. 2 shows the intensity of the magnetic field, measured by a gauss meter, versus throat radius.
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Intensity of magnetic field (G)
1400 1200 1000 800 600 400 200
10 20 30 Throat radius (mm)
40
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Fig. 2- magnetic field intensity versus throat radius
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3. Results and discussion 3.1 Droplet distribution
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A narrow sampling tube connected to the vacuum pump and a small cyclone has been used for measuring the droplet distribution in the throat. The sample of the liquid phase was taken across
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the cross-section of the throat outlet in 8 radial positions. The local flux of the droplets is
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calculated using Eq. 1-3.
(1)
(2) In this equation, na is the local flux (
), mtube is the mass of collected droplets (kg), ATube is
the cross section area of the sampling tube (m2), and t is collecting time (s). The average flux of droplets Na (
) can be calculated using equation 2. m0 is the mass flow rate of the injected
fluid (kg/s), and Aventuri is the venturi cross section at the point in which the sampling tube is located (m2). Therefore, the droplet distribution or the normalized flux (R) can be considered as follows: 9
Journal Pre-proof (3) Fig. 3 shows the radial distribution of normalized flux at the end of the throat. The gas throat velocity is 50 m/s and the liquid flow rate is 1 L/min. The effect of using nanofluid (water/Fe3O4 0.01%) and the magnetic field also is shown in this figure. The slope of each graph is a measure of the uniformity of the droplet distribution. The lower slope indicates a more uniform droplet distribution. 5 4
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y = -0.0805x + 3.4208
3
-p
y = -0.0594x + 3.0329 2
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1 0 10
20 Radial distance (mm)
na
0
30
Water-with magnetic field Nanofluid-with magnetic field Linear (Water-without magnetic field)
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Normalized flux R
Water-without magnetic field
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y = -0.1158x + 4.1529
Linear (Water-with magnetic field) 40
Linear (Nanofluid-with magnetic field)
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Fig. 3- Radial variation of the normalized flux for Vg = 50 m/s and Q = 1 L/min
It is evident that the magnetic field is active on the distribution of the droplets of both water and nanofluid, and the use of a magnetic field increases the uniformity of the droplets distribution. Due to the magnetic field, droplets are distributed more uniformly across the throat as the slope decreases. Without the magnetic field, the liquid droplets tend to pass through the center region of the throat. As the magnetic field is applied, due to the bipolarity of the water droplets and the magnetic properties of the nanofluid, the droplets are diverted to the tube walls by the magnetic field (which is greater near the walls as shown in Fig. 2). The droplet distribution in water / Fe3O4 nanofluid is improved, due to the greater effectiveness of the magnetic nanoparticles in contact with the magnetic field.
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Journal Pre-proof Another factor, which has a remarkable influence on the distribution of the droplets, is the smaller diameter of the nanofluid droplets. According to the lower surface tension of water / Fe3O4 nanofluid compared to base fluid, smaller droplets are formed and more uniformity of droplets is observed [26].
3.2 Pressure drop
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The pressure drop through the throat was measured using a u-type accurate manometer. Fig. 4 shows the variations of the pressure drop in the throat versus air velocity. Pressure drop under
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different gas velocities with and without liquid injection for water and water/Fe3O4 nanofluid is shown. The liquid flowrates are 1 and 2 L/min in Fig. 4 (a) and (b), respectively and the volume
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fraction of Ferro-nanofluid is 0.01%. As shown in Fig. 4, by increasing the gas velocity, the dry
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pressure drop increases, due to the change in the gas momentum and the friction between the gas and the wall of the venturi's throat. The average dry pressure drop across the throat is about
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25.8% lower than the wet pressure drop (Q=1L/min). This trend has also been reported in other studies, such as Allen and Santen [27].
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It is clear that the pressure drop also depends on the liquid flow rate, as shown in Fig. 4 (a) and (b). A higher liquid flow rate means that more energy is needed to accelerate the droplets of the
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liquid; therefore by increasing the liquid flow rate, the pressure drop increases. Virkar and Sharma [28] reported the same results.
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The effect of the nanofluid and the magnetic field also are shown in Fig. 4. It is clear that the pressure drop of nanofluid is higher than the base fluid, distilled water.
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Pressure drop (pa)
200
(a)
150
100
50
0 25
30
35
40
Air velocity (m/s)
50
55
Nanofluid without magnetic field
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Nanofluid with magnetich field
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Water with magnetic field
Water without magnetic field
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Dry pressure drop
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50 0 20
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Pressure drop (pa)
250
30
35
40
45
50
55
Air velocity (m/s)
Nanofluid with magnetich field
Nanofluid without magnetic field
Water with magnetic field
Water without magnetic field
Dry pressure drop
Fig. 4- Effect of gas velocity and magnetic field on dry and wet pressure drop (a) Q=1L/min, (b) Q=2L/min As the Fe3O4/water nanofluid concentration increases, the density and viscosity increases, while the surface tension decreases [29]. Consequently, with reducing the surface tension, smaller droplets were formed, causing the droplets to get more kinetic energy when accompanied by 12
Journal Pre-proof more pressure drop of gas. For instance, according to Fig. 4 (b), the pressure drop of nanofluid under the constant velocity of Vg= 30 m/s is 33%% higher than that for distilled water. Regarding the influence of the magnetic field on the pressure drop, it is clear that using the magnetic field in both distilled water and nanofluid led to an increase in the pressure drop. For instance, according to Fig. 4 (b), in a system with the magnetic field and Vg=40 m/s, the enhancement in pressure drop in water and nanofluid is 50% and 64%, respectively. The reason for this enhancement can be the diameter contraction of the liquid droplets in the presence of the
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magnetic field, as described in the previous section.
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Fig. 5 (a) and (b) shows the pressure loss within the converging, throat, and diverging sections. The effects of gas velocity, nanofluid, and magnetic field are shown in this figure and the trends
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are the same as Fig. 4.
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As shown in Fig. 5 (a) and (b), in the converging section, most of the pressure drop is due to the acceleration of the gas. Friction also increases the pressure drop in the throat. In the divergence
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both distilled water and nanofluid.
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section, the pressure drop is partially recovered by up to 70% [30]. This behavior is the same for
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(a) 1400
pressure drop (pa)
1200 with magnetic field V=30m/s
1000 800 600
without magnetic field V=30m/s
400
with magnetic field V=50 m/s
200
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without magnetic field V=50 m/s
0 0
20
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60
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distance (cm)
100
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(b) 1400
lP
1000
without magnetic field V=30m/s
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800
with magnetic filed V=30 m/s
600 400
with magnetic field V=50m/s
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pressure drop (pa)
1200
0 0
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200 20
40
without magnetic field V=50 m/s 60
80
100
distance (cm)
Fig. 5- Pressure variation for water and nanofluid within venturi scrubber (a) distilled water (b) water/Fe3O4 nanofluids 0.01% V/V, Q=1L/min
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Journal Pre-proof 3.3 Absorption of CO2 3.3.1 Removal efficiency without magnetic field Fig. 6 shows the average S/N ratio and CO2 removal percentage versus four effective factors. The effect of gas velocity, liquid flowrate, nanofluid concentration, and CO2 flowrate on the CO2 removal efficiency is shown. The removal percentage can be calculated using the following relation:
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(4)
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Which, Cin is CO2 concentration at venturi inlet (mol/L) and C0 is CO2 concentration at the
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venturi outlet (mol/L).
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Fig 6-Effects of important factors on the CO2 removal efficiency and average S/N ratio (without magnetic field) It is clear that the enhancement of all four parameters has a positive effect on the CO2 absorption. These observations were consistent with other studies such as Gamisans et al. [10]. By enhancing 16
Journal Pre-proof the gas velocity, the waves formed will be stronger and higher at the surface of the liquid jet, which increases the droplet break up rate. Hence, spray formation began with increasing the gas velocity in a shorter distance. As the gas velocity in the throat increases, the liquid droplets break into the smaller droplets, thus, the contact surface between the phases enhances; thereby, the mass transfer rate increases [31-34]. By increasing the CO2 flowrate, in fact, the driving force is increased, and therefore the CO2 removal efficiency is improved. Regarding Fig. 6, the CO2 removal percentage is enhanced by
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% concentration is 25.2% higher than that in distilled water.
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increasing the nanoparticles volume fraction. CO2 removal efficiency in the nanofluid with 0.02
Brownian motion of nanoparticles and grazing effect are two important mechanisms, which
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increase the mass transfer rate of nanofluids. By adding nanoparticles in the base fluid, the micro convection caused by the Brownian motions of the nanoparticles increases the mass transfer rate.
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The grazing effect is related to the gas adsorption in the layer around the nanoparticles in the
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gas-liquid interface [1].
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3.3.2 Removal efficiency with the magnetic field In this section, the CO2 removal efficiency in the venturi scrubber equipped with a non-uniform
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magnetic field is investigated. The experiments were carried out based on four factors of throat
3.
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gas velocity, liquid flow rate, nanoparticle volume fraction, and CO2 flow rate, according to table
The results are shown in Fig. 7. By comparing Figs. 6 and 7, it is evident that using the magnetic field in all experiments improved the removal percentage. By placing a Ferro-fluid (Fe3O4/water nanofluid) alongside the magnetic field, a regular pattern of peaks-valleys is created on the surface of the nanofluid. This effect, which is known as the Rosensweig or normal-field instability, will make the surface wavy, therefore the surface free energy and turbulence of the liquid increases [35]. The turbulence of the liquid flow improves the mass transfer performance. By increasing the gas and liquid flowrate, the removal efficiency increases. It can be found that the removal efficiency of nanofluid is higher than that in the base fluid. However, the effect of nanofluid concentration on the removal efficiency under the magnetic field differs from that 17
Journal Pre-proof without the magnetic field. By increasing the nanofluid concentration to an optimal point, the removal efficiency increases, and then decreases. The optimum volume fraction of magnetic nanofluid is 0.001 %, which the enhancement of removal efficiency in comparison with distilled water is 33%. As mentioned earlier, this enhancement is due to the grazing effect and Brownian motion. However, the reduction of removal rate in high concentrations is might be due to nanoparticles agglomeration, nanoparticle settling and nanofluid instability in the presence of the magnetic field. Philip et al. [36] obtained similar results by considering Fe3O4/water nanofluid. In a magnetic nanofluid, each magnetite nanoparticle is a single domain superparamagnetic with
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a magnetic moment, and the magnetic nanoparticles form chainlike structure. Under absent of
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the magnetic field, the magnetic moments are oriented in random direction. By increasing the magnetic intensity, the moments of the nanoparticles start to align themselves along the direction
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of the magnetic field. Increasing the nanofluid volume fraction leads to the formation of chains extending to the entire volume of the sample cell. The reduction of mass transfer in high
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concentrations is expected to be due to the cross-linking of the chains and distortion in the
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nematic-like order.
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Fig. 7- Effects of important factors on the CO2 removal efficiency and average S/N ratio (with magnetic field) The analysis of variance, ANOVA, is presented in Table 4. ANOVA is used to determine the relative importance of various factors on the experiment response (CO2 removal efficiency). In ANOVA analysis, the values of freedom degree (DOF), sum of squares, variance, F-Value, P19
Journal Pre-proof value, and percent of contribution (P, %) for each factors in response are obtained. It can be observed that the gas velocity in the throat has the most contribution (36.89%) among the factors in the CO2 removal efficiency and the error percent in the response is very low (3.06%). Given the values, the degree of freedom for each factor is three. According to the data reported in table 4, all of the factors considered efficacious; besides, the error value is less than 4%. Table 4- Analysis of variance for CO2 removal efficiency DOF
Adj SS
Adj MS
F-Value
P-value
P(%)
Gas Throat
3
18.202
54.607
12.05
0.035
36.89
Liquid Flow Rate
3
9.337
28.012
6.18
0.084
18.92
Nanofluid
3
10.916
32.747
7.23
0.069
22.12
CO2 Flow Rate
3
9.374
28.122
6.21
0.084
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Error
3
1.511
Total
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Velocities (m/s)
of
Factors
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Concentration
3.06
na
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4.532
Further experiments were carried out for a closer look at the effect of the magnetic field on the
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CO2 absorption in magnetic nanofluids. The confirmation study after getting the optimized conditions also are conducted to verifying the experimental design results based on Taguchi
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method. According to the Taguchi data and analysis of variance, it was found that the optimum operating conditions for maximum carbon dioxide absorption in the nanofluid are Vg = 50m/s, Ql = 1.5 L/min, and QCO2 = 5 L/min. The effect of nanofluid volume fraction and magnetic field on the removal percentage of CO2 under the optimum conditions is shown in Fig. 8.
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Removal percentage %
20
Without Magnetic Field
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With Magnetic Field 10
0
0.005
0.01
of
5 0.015
0.02
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Nanofluid volume fraction
0.025
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Fig. 8- Effect of nanofluid volume fraction and magnetic field on the removal percentage of CO2
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(Vg=50 m/s,Ql=1.5 L/min,QCO2=5 L/min)
It is clear the magnetic field enhanced the removal percentage of CO2; besides, the absorption
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rate of CO2 in nanofluid is higher than that in the base fluid. For instance, in 0.02% nanofluid without magnetic field, the removal rate is 20% higher than that in the pure water; also, the
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average removal rate of nanofluid under the magnetic field is about 26.2% higher than removal rate without magnetic field.
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Another matter that is clear from this figure is that the optimum volume fraction of nanofluid is 0.001, which confirms the experimental design results presented in Fig.7. After this point, in the absence of a magnetic field, the increase in removal rate remains almost constant, and under the magnetic field, the removal rate decreases. The decrease in removal rate after volume fractions of 0.001 may be due to the deposition of nanoparticles and instability of dense nanofluid in the presence of the magnetic field.
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Journal Pre-proof 4. Conclusion The purpose of this work was to study the effect of Fe3O4/water nanofluid on CO2 removal efficiency in a venturi scrubber in the absence and presence of an external magnetic field. The absorption rate, pressure drop and droplet distribution were investigated. The results showed that the droplet distribution in Fe3O4/water nanofluid was more uniform in comparison with distilled water, and the use of the magnetic field increases the uniformity of the distribution of the droplets. Although the distribution of the droplets was improved using the magnetic field, but the
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pressure drop was increased significantly. Besides, the pressure drop increased by increasing the nanofluid concentration. The results showed that the optimum volume fraction of nanofluid was
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0.001 vol%, which compared to pure water, increased the removal efficiency in the absence and
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in the presence of the magnetic field by up to 19.6% and 36.3%, respectively.
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Journal Pre-proof CRediT author statement Nastaran Abbaspour: Conceptualization, Software Masoud Haghshenasfard: Supervision, Writing- Original draft preparation, Methodology Mohammadreza Talaei: Conceptualization, Visualization, Investigation.
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Hasanali Amini: Resources
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Journal Pre-proof Highlights: Magnetic field is an effective parameter on CO2 absorption rate and pressure drop
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Droplet distribution in nanofluid is more uniform in comparison with pure water
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Use of magnetic field increases the uniformity of the droplets distribution
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The pressure drop increases by increasing the nanofluid concentration
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29