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Dust particle removal efficiency of a venturi scrubber
Annals of Nuclear Energy 54 (2013) 178–183
Contents lists available at SciVerse ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Dust particle removal efficiency of a venturi scrubber Majid Ali ⇑, Changqi Yan, Zhongning Sun, Haifeng Gu, Khurram Mehboob College of Nuclear Science and Technology, Harbin Engineering University, Harbin, China
a r t i c l e
i n f o
Article history: Received 2 April 2012 Received in revised form 15 September 2012 Accepted 11 November 2012 Available online 13 December 2012 Keywords: Venturi scrubber Nuclear power plant Filtered vented containment system Impaction Removal efficiency
a b s t r a c t The venturi scrubber is one of the most efficient gas cleaning devices to remove the contaminated particles from gaseous stream during severe accident in nuclear power plant. This study is focused on the dust particle removal efficiency of the venturi scrubber experimentally and theoretically. The venturi scrubber encapsulates the dust particles in petite water droplets flowing into it. The water injected into the scrubber is in the form of water film. The study investigates the removal efficiency of venturi scrubber for throat gas velocities of 130, 165 and 200 m/s and liquid flow rates 0.3–1 m3/h, whereas dust concentration ranges between 0.1 and 1 g/m3. The hydrophobic titanium dioxide (TiO2) particles having density 4.23 g/cm3 and mean diameter of 1 lm are used as dust particles in this research. Filtration technique is used to measure the concentration of dust particles at inlet and outlet. Experimental results show that the removal efficiency is higher with the increase of throat gas velocity and liquid flow rate. A mathematical model is employed for the verification of experimental results. The model concurs well with the experimental results. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Severe accident in commercial Nuclear Power Plant (NPP) such as Fukushima Daiichi Nuclear Power Station on 11 March 2011 activated by an earthquake and tsunami cause’s devastation at large level in Japan. The hydrogen produced due to failure of cooling systems of reactor core results in excessive pressure buildup inside the containment. The containment of Fukushima Daiichi NPP was broken due to hydrogen explosion. As a result, the large amount of radionuclides was escaped from the containment (Shozugawa et al., 2012). This problem raises the concern of containment integrity of NPP in severe accident. In the light of this scenario, Filtered Containment Venting System (FCVS) is necessary to install in working and under construction NPP to perform the following task (Rust et al., 1995); To protect the integrity of containment of NPP against excessive pressure buildup. To retain the aerosol, iodine, and organic compounds (WGAMA, 2007) from the radioactive exhaust gas. Mostly FVCS installed in NPP are wet scrubbers which are double-staged consist of venturi scrubber and metallic fiber filter (Rust et al., 1995; Kolditz, 1995; Reim and Hurlebaus, 1995). In the first stage, venturi scrubber successfully removes submicron particles ⇑ Corresponding author. Tel./fax: +86 451 82569655. E-mail addresses: [email protected] (M. Ali), [email protected] (C. Yan), [email protected] (Z. Sun), [email protected] (H. Gu), [email protected] (K. Mehboob). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.11.005
and gaseous pollutants simultaneously from the gas stream via droplets within short contact time between the liquid and gas phases. The submicron particles are captured while the gaseous pollutants are absorbed by the liquid. In the second stage, the filter unit retains the aerosol particles that are usually too small and remove water droplets from carrier gas (AREVA INC., 2011). Nowadays, numerous kinds of technologies are available for gas cleaning used in different industries and power plant such as cyclones, settling chambers, fabric filters, electrostatic precipitators, and wet scrubbers. Wet scrubber is one of the most efficient device uses liquid scrubbing technique (LST) to remove particles and gaseous contaminants from exhaust gas. Wet scrubber is further categorized into different scrubbers having different configuration like spray tower scrubbers, mechanically aided scrubbers, orifice scrubbers, packed bed scrubbers, ionizing wet scrubbers, fiber bed scrubbers, moving bed scrubbers, tray scrubbers, catenary grid scrubbers, condensation growth scrubbers, rod deck scrubbers, collision scrubbers, ejector scrubbers, and venturi scrubbers (US EPA, 1999; Mi and Yu, 2012). Among all of these, venturi scrubbers are a highly efficient gas-cleaning device that uses a liquid in the form of droplets to remove dust particles of diameter range between 0.1 and 300 lm (Tri-Mer Co., 2012) and absorb gaseous contaminants like SO2, I2, and CH3I, etc. from gaseous stream (Wei-yi et al., 2011; Rust et al., 1995). Economopoulou highlights some advantages of Venturi scrubber which increases its worth and importance (Economopoulou and Harrison, 2007). It is relatively simple. It occupies less space. It has no rotating part.
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
179
Nomenclature a C Cd d E g Q Re t v x
acceleration (m/s2) dust concentration (g/m3) drag coefficient (dimensionless) diameter (m) efficiency (dimensionless) acceleration due to gravity (m/s2) volumetric flow rate (m3/h) Reynolds number (dimensionless) time of flight of droplet (s) velocity (m/s) length (m)
Greek letters g inertial impaction parameter (dimensionless) l viscosity (N s/m2)
It It It It
has compact volume. is easy to install and maintain. can handle high-temperature and corrosive gases. can remove dust and gaseous pollutants simultaneously.
Literature survey reveals that the collection efficiency of venturi scrubbers is a function of many variables, such as the droplet size, particle size, gas velocity, liquid flow rate and the liquid injection system. The collection efficiency of a venturi scrubber is affected by numerous phenomenons operating simultaneously. These include inertial impaction, interception, diffusion, nucleation and growth, and condensation. The contribution of any one mechanism depends on size of particle and droplet and their relative velocity. Several models have been developed to study the removal efficiencies of venturi scrubber. Various models are easy and simple and have straightforward solutions whereas remaining others are complex and solved numerically. Calvert (1970), Boll (1973), and Yung et al. (1978) models are the most popular models for the study of collection efficiency of venturi scrubber. Calvert (1970) predicted the collection efficiency based on single droplet impaction. The approximation of Walton and Woolcock correlation from experimental results was used to estimate the single droplet collection efficiency. Yung et al. (1978) modified the Calvert model without using empirical constants to measure the single droplet collection efficiency of scrubber. Only inertial impaction parameter was considered in the mathematical model. Water was introduced into the scrubber in three different ways and initial velocity of water droplets was considered to be zero. The obtained results were compared with experimental results of Ekman and Johnstone (1951). It was determined that long throat length increases the efficiency. Literature survey further reveals that the liquid supplied into the venturi scrubber by two methods; forced feed method through pumps and self-priming method based on the pressure difference between hydrostatic pressure of the liquid in tank and static pressure of the gas in venturi scrubber (Mayinger and Lehner, 1995; Lehner, 1998). Lehner (1998) studied the operating condition, liquid crumbling, and collection efficiency of a self-priming venturi. Liquid was introduced in the form of jet. It was observed that the aerosol separation efficiency was enhanced by injecting liquid at multistage. It has been shown that under self-priming operation the liquid loading increases with decreasing gas velocity and vice versa (Mayinger and Lehner, 1995; Lehner, 1998). Lim et al. (2006) studied the collection efficiency of reverse jet venturi scrubber for different sizes of particle. The size of particles was represented by lognormal distribution and moment equations. Liquid was injected in reverse direction in the form of spray to increase
q r w
density (kg/m3) surface tension (N/m) stokes number (dimensionless)
Subscript d di g i l o p r re
droplet initial value of droplet gas inlet liquid outlet particle relative removal
the efficiency by increasing relative velocity, residence time, surface area and dense packing. Impaction, interception and diffusion mechanism to encapsulate the particle in droplet were assumed. It was observed that impaction was dominant due to high relative velocity between droplet and gas. The removal efficiency was higher for droplet having smaller diameter due to higher surface area. The present research focuses on the study of removal efficiency of dust in self-priming venturi scrubber. In this research, the dust removal efficiency is investigated at different throat gas velocity, liquid flow rate, and dust concentrations. Experimental result of dust removal efficiency is correlated with the mathematical model to understand the physical phenomenon involved during removal efficiency and analyze the performance of venturi scrubber. The principal collection mechanism of dust in this venturi scrubber is primarily based on the inertial impaction. Experiments are conducted with throat gas velocity 130, 165, and 200 m/s, liquid flow rate ranges from 0.3 m3/h to 1 m3/h and dust concentration is kept within 0.1–1.0 g/m3.
2. Experimental setup The schematic diagram of experimental setup to study the removal efficiency of a venturi scrubber is shown in Fig. 1. The compressed gas from air compressor is stored in air tank. The gas is filtered with air filters. The gas flow rate is adjusted by a valve. The mass flow rate of gas is measured by a mass flow meter. The direction of gas flow in venturi scrubber is against the gravity. The water is supplied to venturi scrubber from water tank. The water tank is filled at certain height to create requires hydrostatic head effect. A constant water level is maintained for each run. The venturi scrubber is not submerged in a venturi tank. The dust encapsulated droplets are collected in venturi tank. The mean size of titanium dioxide (TiO2) used as a dust particle is 1 lm. The venturi scrubber is operated at throat gas velocity 130, 165, and 200 m/s and liquid flow rate between 0.3 and 1 m3/h, and dust concentration varies from 0.1 g/m3 to 1.0 g/m3. Inlet and outlet concentration of dust particle are measured at sampling point Si and So. The samples are collected by isokinetic sampling, and concentrations at inlet and outlet are measured by filtration technique. The samples of dust are collected at filter paper. Digital mass balance device is used to measure mass of the collected dust sample. In filtration technique, the total mass of the dust particles collected is found by taking the difference between the weight of the filter paper containing the collected dust particles
180
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
Fig. 1. Experimental setup for dust removal efficiency of venturi scrubber.
and the weight of filter paper. The removal efficiency is calculated from the following equation,
Ere ¼
C in C 0 C in
ð1Þ
6. No evaporation and condensation. 7. No coalescence between the droplets. 3.2. Material balance equation
The following venturi scrubber is used in the research as shown in Fig. 2.
The material balance equation for co-flow venturi scrubber is given as (Noel de Nevers, 2000):
3. Mathematical model
mass of particle transferred to drops perunit time perunit ¼
3.1. Assumptions A mathematical model is developed based on following assumptions: 1. 2. 3. 4. 5.
Co-flow direction of gas and droplet. Vertical direction of gas and droplet against gravity. Droplets are spherical. Droplets have same size. Droplets are distributed uniformly.
v olume
ðmass transferred to one dropletÞðNo: of drops=timeÞ Volume of region
By arranging, we get
dC 3 g Ql vd ¼ dx C 2 Dd Q g ðv g v d Þ
ð2Þ
Integrating above equation,
Co 3 Ql x vd ¼ exp g 2 Q g Dd ðv g v d Þ Ci
Fig. 2. Schemtaic diagram of venturi scrubber.
ð3Þ
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
181
Nukiyama and Tanasawa (1938) correlation used to measure the size of droplet,
Dd ¼
0:585
rffiffiffiffiffi
r ll þ 1:683 103 pffiffiffiffiffiffiffiffi ql rql
vr
1:5 0:45 1000Q l Qg
ð4Þ
Velocity of droplet calculated from the equation of motion of droplet (Boll, 1973),
dv d 3lg C D ðv g v d Þ2 ¼ dt 4qd Dd
ð5Þ
Since the flow is against the gravity, therefore,
dv d 3lg C D ðv g v d Þ2 g ¼ dt 4qd Dd
vd ¼ vd
i
þ
Z
t
0
ð6Þ
dv d dt dt
ð7Þ
Drag coefficient is calculated by following equation (Dickinson and Marshall, 1968),
C d ¼ 0:22 þ
24 ð1 þ 0:15Re0:6 Þ Re
ð8Þ
3.3. Single droplet collection efficiency The target efficiency of a single droplet g is calculated from inertial impaction mechanism which is a function of Stokes number. This phenomenon is more dominant for particle size of 1 lm (Lim et al., 2006). The impaction of single droplet efficiency is defined as (Calvert, 1970),
g¼
w w þ 0:7
2 ð9Þ
where w is expressed as (Pak and Chang, 2006),
w¼
qp d2p ðV p V di Þ 9lg dd
ð10Þ
3.4. Dust removal efficiency The dust removal efficiency is calculated from the following equation,
Ere ¼ 1
Co 3 Ql x ¼ 1 exp 2 Q g dd Ci
vd vg vd
g
Fig. 3. Liquid flow rate at different throat gas velocity and hydrostatic head.
The dust removal efficiency of venturi scrubber is the ratio of total concentrations removed to the total concentration at an inlet. In this research, the dust removal efficiency is measured by injecting hydrophobic particle TiO2, with density 4.23 g/cm3 and mean diameter of 1 lm. In Fig. 4 the removal efficiency of dust is analyzed for different liquid flow rate by varying throat gas velocity. The result shows that the removal efficiency increases with the increase of liquid flow rate. At higher gas velocity, the efficiency will be higher. This is due to high relative velocity between gas and droplets in throat section. 4.2. Dust concentration In Fig. 5, removal efficiency is investigated at different dust concentration ranges from 0.1 g/m3 to 1 g/m3. The liquid introduced into the venturi scrubber is in the form of film at the end of convergent section, which is immediately converted into petite droplets due to high kinetic energy of gas developed due to small restriction between convergent and divergent section. As the relative velocity of dust is higher in throat, so it directly hits the droplet and encapsulates it. The trend in Fig. 5 shows the higher efficiency at higher throat gas velocity. The result designates the overall maximum efficiency of 99.5% is achieved for this venturi scrubber with throat gas velocity 200 m/s.
ð11Þ
4. Results and discussion 4.1. Liquid flow rate Lehner (1998) divided the venturi scrubber into two categories based on the liquid supplied: forced feed, and self-priming. In force feed method, the liquid is supplied through pumps such that liquid flow rate is independent of the gas flow rate. But, in self-priming venturi scrubber, the liquid flow rate depends on the gas flow rate. The liquid supplied into self-priming venturi scrubber depends on the static pressure of gas in the throat and hydrostatic pressure of the liquid in the water tank. The liquid flow rate is analyzed at different throat gas velocities. Fig. 3 depicts that with the increase of throat gas velocity decreases liquid flow rate. These trends are similar to the Lehner (1998) in which liquid was injected in jet form through holes. It is also observed that with the increase of hydrostatic head in the tank it increases the flow rate of liquid.
Fig. 4. Removal efficiency of venturi scrubber at different liquid flow rate.
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M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
throat creates a vacuum and pushes the droplet to disperse in it. The distribution of liquid at throat is uniform, and all liquid atomized instantaneously into monodisperse droplets which is calculated from Nukiyama and Tanasawa (1938) correlation. These petite droplets come in the way of dust particles. The dust particle does not follow the gas stream anymore, hit the droplet and finally encapsulates in it. The impaction increases with the increase in relative velocity between particle and dust, and with the decrease of diameter of droplet. The decrease in diameter increases the surface area, and increases the chances of interaction between dust particle and droplet (Lim et al., 2006). Fig. 6 depicts the scatter graph between the predicted and experimental removal efficiency. The model agrees well with the experimental result. 5. Conclusion Based on the work presented above, the following conclusions can be made: Fig. 5. Removal efficiency of venturi scrubber at different dust concentration.
4.3. Model validation A mathematical model is proposed for the verification of experimental results and to enlighten the phenomenon involved in dust removal efficiency. The removal efficiency of a venturi scrubber is a function of various parameters: droplet diameter, fluid velocity, volumetric flow rates and impaction. These factors highly influence the dust removal efficiency of the venturi scrubber. The mechanism involved in capturing of dust particle depends upon size of particle. As the size of dust particle is 1 lm, therefore, single droplet collection efficiency is calculated from impaction mechanism (Lim et al., 2006). However, impaction mechanism depends on characteristics of dust and droplet. The inertial impaction is calculated from Walton and Woolcock (1960) equation which is also used in Calvert (1970) and Yung et al. (1978) models to predict removal efficiency. Stokes number is always greater than one calculated from Eq. (10) which indicates that the particles remain unaffected and continue their trajectory implying, collision with the droplet rather than flowing around it (Fokeer, 2006). The dust particles always pursue the gas stream. The low pressure at the
Fig. 6. Graph between calculated and experimental removal efficiency of venturi scrubber.
Liquid flow rate increases in throat with the increase in hydrostatic head in the tank and decreases with the increase in throat gas velocity. The dust particle removal efficiency increases with the increase of throat gas velocity and liquid flow rate. The maximum dust particle removal efficiency of venturi scrubber is 99.5%, which is measured for gas velocity 200 m/s at throat. The theoretical model of removal efficiency based on inertial impaction fits well with experimental result.
Acknowledgement This work is supported by the College of Nuclear Science and Technology, Harbin Engineering University, Harbin, China. References AREVA INC., 2011. Filtered Containment Venting System. Boll, H., 1973. Particle collection and pressure drop in venturi scrubber. Ind. Eng. Chem. Fundam. 12 (1), 40–49. Calvert, S., 1970. Venturi and other atomizing scrubbers efficiency and pressure drop. AlChE J. 16 (3), 392–396. Dickinson, D.R., Marshall, W.R., 1968. The rates of evaporation of sprays. AlChE J. 14, 541. Economopoulou, A.A., Harrison, R.M., 2007. Graphical analysis of the performance of venturi scrubbers for particle abatement. Part I: Rapid collection efficiency evaluation. Aerosol Sci. Technol. 41, 51–62. Ekman, F.O., Johnstone, H.F., 1951. Collection of aerosols in a venturi scrubber. Ind. Eng. Chem. 43 (6), 1358–1363. Fokeer, S., 2006. An Investigation of Geometrically induced Swirl Applied to Lean Phase Pneumatic Flows. University of Nottingham, Ph.D. Thesis. Kolditz, J., 1995. GKN I and GKN II: Reactor containment pressure relief and measures against hydrogen energy release in case of severe accidents. Nucl. Eng. Des. 157, 299–310. Lehner, M., 1998. Aerosol separation efficiency of a venturi scrubber working in self-priming mode. Aerosol Sci. Technol. 28, 389–402. Lim, K.S. et al., 2006. Prediction for particle removal efficiency of a reverse jet scrubber. J. Aerosol Sci. 37, 1826–1839. Mayinger, F., Lehner, M., 1995. Operating results and aerosol deposition of a venturi scrubber in self-priming operation. Chem. Eng. Process 34, 283–288. Mi, T., Yu, X.M., 2012. Dust removal and desulphurization in a novel venturi scrubber. Chem. Eng. Process., http://dx.doi.org/10.1016/j.cep.2012.07.010. Nevers, N.D., 2000. Air Pollution Control Engineering, second ed. McGraw Hill, New York. Nukiyama, S., Tanasawa, Y., 1938. An experiment on the atomisation of liquid by means of air stream. Trans. Soc. Mech. Eng. (Japan) 4, 86–93. Pak, S.I., Chang, K.S., 2006. Performance estimation of a venturi scrubber using a computational model for capturing dust particles with liquid spray. J. Hazard. Mater. 138(B), 560–573. Reim, W., Hurlebaus, D., 1995. Protection of the containment of the twin BWR KRB II plant against overpressure failure during severe accidents. Nucl. Eng. Des. 157, 321–335.
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183 Rust, H. et al., 1995. Pressure release of containments during severe accidents in Switzerland. Nucl. Eng. Des. 157, 337–352. Shozugawa, K., Nogawa, N., Matsuo, M., 2012. Deposition of fission and activation products after the Fukushima Daiichi nuclear power plant accident. Environ. Pollut. 163, 243–247. Tri-Mer Co., 2012. http://www.tri-mer.com/venturi-scrubbers.html. US EPA, 1999. APTI 413: Control of Particulate Matter Emissions, fifth ed. http:// www.epa.gov/eogapti1/catalog/cc413.html.
183
Walton, W.H., Woolcock, A., 1960. The suppression of air borne dust by water spray. Int. J. Air Pollution 3, 129. Wei-yi, S. et al., 2011. Simultaneous absorption of NOx and SO2 from flue gas with pyrolusite slurry combined with gas-phase oxidation of NO using ozone. J. Hazard. Mater. 192, 124–130. WGAMA, 2007. State of the Art Report on Iodine Chemistry NEA/CSNI/R(1). Yung, S.C. et al., 1978. Venturi scrubber performance model. Environ. Sci. Technol. 12 (4), 456–459.
Contents lists available at SciVerse ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Dust particle removal efficiency of a venturi scrubber Majid Ali ⇑, Changqi Yan, Zhongning Sun, Haifeng Gu, Khurram Mehboob College of Nuclear Science and Technology, Harbin Engineering University, Harbin, China
a r t i c l e
i n f o
Article history: Received 2 April 2012 Received in revised form 15 September 2012 Accepted 11 November 2012 Available online 13 December 2012 Keywords: Venturi scrubber Nuclear power plant Filtered vented containment system Impaction Removal efficiency
a b s t r a c t The venturi scrubber is one of the most efficient gas cleaning devices to remove the contaminated particles from gaseous stream during severe accident in nuclear power plant. This study is focused on the dust particle removal efficiency of the venturi scrubber experimentally and theoretically. The venturi scrubber encapsulates the dust particles in petite water droplets flowing into it. The water injected into the scrubber is in the form of water film. The study investigates the removal efficiency of venturi scrubber for throat gas velocities of 130, 165 and 200 m/s and liquid flow rates 0.3–1 m3/h, whereas dust concentration ranges between 0.1 and 1 g/m3. The hydrophobic titanium dioxide (TiO2) particles having density 4.23 g/cm3 and mean diameter of 1 lm are used as dust particles in this research. Filtration technique is used to measure the concentration of dust particles at inlet and outlet. Experimental results show that the removal efficiency is higher with the increase of throat gas velocity and liquid flow rate. A mathematical model is employed for the verification of experimental results. The model concurs well with the experimental results. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Severe accident in commercial Nuclear Power Plant (NPP) such as Fukushima Daiichi Nuclear Power Station on 11 March 2011 activated by an earthquake and tsunami cause’s devastation at large level in Japan. The hydrogen produced due to failure of cooling systems of reactor core results in excessive pressure buildup inside the containment. The containment of Fukushima Daiichi NPP was broken due to hydrogen explosion. As a result, the large amount of radionuclides was escaped from the containment (Shozugawa et al., 2012). This problem raises the concern of containment integrity of NPP in severe accident. In the light of this scenario, Filtered Containment Venting System (FCVS) is necessary to install in working and under construction NPP to perform the following task (Rust et al., 1995); To protect the integrity of containment of NPP against excessive pressure buildup. To retain the aerosol, iodine, and organic compounds (WGAMA, 2007) from the radioactive exhaust gas. Mostly FVCS installed in NPP are wet scrubbers which are double-staged consist of venturi scrubber and metallic fiber filter (Rust et al., 1995; Kolditz, 1995; Reim and Hurlebaus, 1995). In the first stage, venturi scrubber successfully removes submicron particles ⇑ Corresponding author. Tel./fax: +86 451 82569655. E-mail addresses: [email protected] (M. Ali), [email protected] (C. Yan), [email protected] (Z. Sun), [email protected] (H. Gu), [email protected] (K. Mehboob). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.11.005
and gaseous pollutants simultaneously from the gas stream via droplets within short contact time between the liquid and gas phases. The submicron particles are captured while the gaseous pollutants are absorbed by the liquid. In the second stage, the filter unit retains the aerosol particles that are usually too small and remove water droplets from carrier gas (AREVA INC., 2011). Nowadays, numerous kinds of technologies are available for gas cleaning used in different industries and power plant such as cyclones, settling chambers, fabric filters, electrostatic precipitators, and wet scrubbers. Wet scrubber is one of the most efficient device uses liquid scrubbing technique (LST) to remove particles and gaseous contaminants from exhaust gas. Wet scrubber is further categorized into different scrubbers having different configuration like spray tower scrubbers, mechanically aided scrubbers, orifice scrubbers, packed bed scrubbers, ionizing wet scrubbers, fiber bed scrubbers, moving bed scrubbers, tray scrubbers, catenary grid scrubbers, condensation growth scrubbers, rod deck scrubbers, collision scrubbers, ejector scrubbers, and venturi scrubbers (US EPA, 1999; Mi and Yu, 2012). Among all of these, venturi scrubbers are a highly efficient gas-cleaning device that uses a liquid in the form of droplets to remove dust particles of diameter range between 0.1 and 300 lm (Tri-Mer Co., 2012) and absorb gaseous contaminants like SO2, I2, and CH3I, etc. from gaseous stream (Wei-yi et al., 2011; Rust et al., 1995). Economopoulou highlights some advantages of Venturi scrubber which increases its worth and importance (Economopoulou and Harrison, 2007). It is relatively simple. It occupies less space. It has no rotating part.
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
179
Nomenclature a C Cd d E g Q Re t v x
acceleration (m/s2) dust concentration (g/m3) drag coefficient (dimensionless) diameter (m) efficiency (dimensionless) acceleration due to gravity (m/s2) volumetric flow rate (m3/h) Reynolds number (dimensionless) time of flight of droplet (s) velocity (m/s) length (m)
Greek letters g inertial impaction parameter (dimensionless) l viscosity (N s/m2)
It It It It
has compact volume. is easy to install and maintain. can handle high-temperature and corrosive gases. can remove dust and gaseous pollutants simultaneously.
Literature survey reveals that the collection efficiency of venturi scrubbers is a function of many variables, such as the droplet size, particle size, gas velocity, liquid flow rate and the liquid injection system. The collection efficiency of a venturi scrubber is affected by numerous phenomenons operating simultaneously. These include inertial impaction, interception, diffusion, nucleation and growth, and condensation. The contribution of any one mechanism depends on size of particle and droplet and their relative velocity. Several models have been developed to study the removal efficiencies of venturi scrubber. Various models are easy and simple and have straightforward solutions whereas remaining others are complex and solved numerically. Calvert (1970), Boll (1973), and Yung et al. (1978) models are the most popular models for the study of collection efficiency of venturi scrubber. Calvert (1970) predicted the collection efficiency based on single droplet impaction. The approximation of Walton and Woolcock correlation from experimental results was used to estimate the single droplet collection efficiency. Yung et al. (1978) modified the Calvert model without using empirical constants to measure the single droplet collection efficiency of scrubber. Only inertial impaction parameter was considered in the mathematical model. Water was introduced into the scrubber in three different ways and initial velocity of water droplets was considered to be zero. The obtained results were compared with experimental results of Ekman and Johnstone (1951). It was determined that long throat length increases the efficiency. Literature survey further reveals that the liquid supplied into the venturi scrubber by two methods; forced feed method through pumps and self-priming method based on the pressure difference between hydrostatic pressure of the liquid in tank and static pressure of the gas in venturi scrubber (Mayinger and Lehner, 1995; Lehner, 1998). Lehner (1998) studied the operating condition, liquid crumbling, and collection efficiency of a self-priming venturi. Liquid was introduced in the form of jet. It was observed that the aerosol separation efficiency was enhanced by injecting liquid at multistage. It has been shown that under self-priming operation the liquid loading increases with decreasing gas velocity and vice versa (Mayinger and Lehner, 1995; Lehner, 1998). Lim et al. (2006) studied the collection efficiency of reverse jet venturi scrubber for different sizes of particle. The size of particles was represented by lognormal distribution and moment equations. Liquid was injected in reverse direction in the form of spray to increase
q r w
density (kg/m3) surface tension (N/m) stokes number (dimensionless)
Subscript d di g i l o p r re
droplet initial value of droplet gas inlet liquid outlet particle relative removal
the efficiency by increasing relative velocity, residence time, surface area and dense packing. Impaction, interception and diffusion mechanism to encapsulate the particle in droplet were assumed. It was observed that impaction was dominant due to high relative velocity between droplet and gas. The removal efficiency was higher for droplet having smaller diameter due to higher surface area. The present research focuses on the study of removal efficiency of dust in self-priming venturi scrubber. In this research, the dust removal efficiency is investigated at different throat gas velocity, liquid flow rate, and dust concentrations. Experimental result of dust removal efficiency is correlated with the mathematical model to understand the physical phenomenon involved during removal efficiency and analyze the performance of venturi scrubber. The principal collection mechanism of dust in this venturi scrubber is primarily based on the inertial impaction. Experiments are conducted with throat gas velocity 130, 165, and 200 m/s, liquid flow rate ranges from 0.3 m3/h to 1 m3/h and dust concentration is kept within 0.1–1.0 g/m3.
2. Experimental setup The schematic diagram of experimental setup to study the removal efficiency of a venturi scrubber is shown in Fig. 1. The compressed gas from air compressor is stored in air tank. The gas is filtered with air filters. The gas flow rate is adjusted by a valve. The mass flow rate of gas is measured by a mass flow meter. The direction of gas flow in venturi scrubber is against the gravity. The water is supplied to venturi scrubber from water tank. The water tank is filled at certain height to create requires hydrostatic head effect. A constant water level is maintained for each run. The venturi scrubber is not submerged in a venturi tank. The dust encapsulated droplets are collected in venturi tank. The mean size of titanium dioxide (TiO2) used as a dust particle is 1 lm. The venturi scrubber is operated at throat gas velocity 130, 165, and 200 m/s and liquid flow rate between 0.3 and 1 m3/h, and dust concentration varies from 0.1 g/m3 to 1.0 g/m3. Inlet and outlet concentration of dust particle are measured at sampling point Si and So. The samples are collected by isokinetic sampling, and concentrations at inlet and outlet are measured by filtration technique. The samples of dust are collected at filter paper. Digital mass balance device is used to measure mass of the collected dust sample. In filtration technique, the total mass of the dust particles collected is found by taking the difference between the weight of the filter paper containing the collected dust particles
180
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
Fig. 1. Experimental setup for dust removal efficiency of venturi scrubber.
and the weight of filter paper. The removal efficiency is calculated from the following equation,
Ere ¼
C in C 0 C in
ð1Þ
6. No evaporation and condensation. 7. No coalescence between the droplets. 3.2. Material balance equation
The following venturi scrubber is used in the research as shown in Fig. 2.
The material balance equation for co-flow venturi scrubber is given as (Noel de Nevers, 2000):
3. Mathematical model
mass of particle transferred to drops perunit time perunit ¼
3.1. Assumptions A mathematical model is developed based on following assumptions: 1. 2. 3. 4. 5.
Co-flow direction of gas and droplet. Vertical direction of gas and droplet against gravity. Droplets are spherical. Droplets have same size. Droplets are distributed uniformly.
v olume
ðmass transferred to one dropletÞðNo: of drops=timeÞ Volume of region
By arranging, we get
dC 3 g Ql vd ¼ dx C 2 Dd Q g ðv g v d Þ
ð2Þ
Integrating above equation,
Co 3 Ql x vd ¼ exp g 2 Q g Dd ðv g v d Þ Ci
Fig. 2. Schemtaic diagram of venturi scrubber.
ð3Þ
M. Ali et al. / Annals of Nuclear Energy 54 (2013) 178–183
181
Nukiyama and Tanasawa (1938) correlation used to measure the size of droplet,
Dd ¼
0:585
rffiffiffiffiffi
r ll þ 1:683 103 pffiffiffiffiffiffiffiffi ql rql
vr
1:5 0:45 1000Q l Qg
ð4Þ
Velocity of droplet calculated from the equation of motion of droplet (Boll, 1973),
dv d 3lg C D ðv g v d Þ2 ¼ dt 4qd Dd
ð5Þ
Since the flow is against the gravity, therefore,
dv d 3lg C D ðv g v d Þ2 g ¼ dt 4qd Dd
vd ¼ vd
i
þ
Z
t
0
ð6Þ
dv d dt dt
ð7Þ
Drag coefficient is calculated by following equation (Dickinson and Marshall, 1968),
C d ¼ 0:22 þ
24 ð1 þ 0:15Re0:6 Þ Re
ð8Þ
3.3. Single droplet collection efficiency The target efficiency of a single droplet g is calculated from inertial impaction mechanism which is a function of Stokes number. This phenomenon is more dominant for particle size of 1 lm (Lim et al., 2006). The impaction of single droplet efficiency is defined as (Calvert, 1970),
g¼
w w þ 0:7
2 ð9Þ
where w is expressed as (Pak and Chang, 2006),
w¼
qp d2p ðV p V di Þ 9lg dd
ð10Þ
3.4. Dust removal efficiency The dust removal efficiency is calculated from the following equation,
Ere ¼ 1
Co 3 Ql x ¼ 1 exp 2 Q g dd Ci
vd vg vd
g
Fig. 3. Liquid flow rate at different throat gas velocity and hydrostatic head.
The dust removal efficiency of venturi scrubber is the ratio of total concentrations removed to the total concentration at an inlet. In this research, the dust removal efficiency is measured by injecting hydrophobic particle TiO2, with density 4.23 g/cm3 and mean diameter of 1 lm. In Fig. 4 the removal efficiency of dust is analyzed for different liquid flow rate by varying throat gas velocity. The result shows that the removal efficiency increases with the increase of liquid flow rate. At higher gas velocity, the efficiency will be higher. This is due to high relative velocity between gas and droplets in throat section. 4.2. Dust concentration In Fig. 5, removal efficiency is investigated at different dust concentration ranges from 0.1 g/m3 to 1 g/m3. The liquid introduced into the venturi scrubber is in the form of film at the end of convergent section, which is immediately converted into petite droplets due to high kinetic energy of gas developed due to small restriction between convergent and divergent section. As the relative velocity of dust is higher in throat, so it directly hits the droplet and encapsulates it. The trend in Fig. 5 shows the higher efficiency at higher throat gas velocity. The result designates the overall maximum efficiency of 99.5% is achieved for this venturi scrubber with throat gas velocity 200 m/s.
ð11Þ
4. Results and discussion 4.1. Liquid flow rate Lehner (1998) divided the venturi scrubber into two categories based on the liquid supplied: forced feed, and self-priming. In force feed method, the liquid is supplied through pumps such that liquid flow rate is independent of the gas flow rate. But, in self-priming venturi scrubber, the liquid flow rate depends on the gas flow rate. The liquid supplied into self-priming venturi scrubber depends on the static pressure of gas in the throat and hydrostatic pressure of the liquid in the water tank. The liquid flow rate is analyzed at different throat gas velocities. Fig. 3 depicts that with the increase of throat gas velocity decreases liquid flow rate. These trends are similar to the Lehner (1998) in which liquid was injected in jet form through holes. It is also observed that with the increase of hydrostatic head in the tank it increases the flow rate of liquid.
Fig. 4. Removal efficiency of venturi scrubber at different liquid flow rate.
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throat creates a vacuum and pushes the droplet to disperse in it. The distribution of liquid at throat is uniform, and all liquid atomized instantaneously into monodisperse droplets which is calculated from Nukiyama and Tanasawa (1938) correlation. These petite droplets come in the way of dust particles. The dust particle does not follow the gas stream anymore, hit the droplet and finally encapsulates in it. The impaction increases with the increase in relative velocity between particle and dust, and with the decrease of diameter of droplet. The decrease in diameter increases the surface area, and increases the chances of interaction between dust particle and droplet (Lim et al., 2006). Fig. 6 depicts the scatter graph between the predicted and experimental removal efficiency. The model agrees well with the experimental result. 5. Conclusion Based on the work presented above, the following conclusions can be made: Fig. 5. Removal efficiency of venturi scrubber at different dust concentration.
4.3. Model validation A mathematical model is proposed for the verification of experimental results and to enlighten the phenomenon involved in dust removal efficiency. The removal efficiency of a venturi scrubber is a function of various parameters: droplet diameter, fluid velocity, volumetric flow rates and impaction. These factors highly influence the dust removal efficiency of the venturi scrubber. The mechanism involved in capturing of dust particle depends upon size of particle. As the size of dust particle is 1 lm, therefore, single droplet collection efficiency is calculated from impaction mechanism (Lim et al., 2006). However, impaction mechanism depends on characteristics of dust and droplet. The inertial impaction is calculated from Walton and Woolcock (1960) equation which is also used in Calvert (1970) and Yung et al. (1978) models to predict removal efficiency. Stokes number is always greater than one calculated from Eq. (10) which indicates that the particles remain unaffected and continue their trajectory implying, collision with the droplet rather than flowing around it (Fokeer, 2006). The dust particles always pursue the gas stream. The low pressure at the
Fig. 6. Graph between calculated and experimental removal efficiency of venturi scrubber.
Liquid flow rate increases in throat with the increase in hydrostatic head in the tank and decreases with the increase in throat gas velocity. The dust particle removal efficiency increases with the increase of throat gas velocity and liquid flow rate. The maximum dust particle removal efficiency of venturi scrubber is 99.5%, which is measured for gas velocity 200 m/s at throat. The theoretical model of removal efficiency based on inertial impaction fits well with experimental result.
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