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Effect of geometry of hydrodynamically cavitating device on degradation of orange-G
Ultrasonics Sonochemistry 20 (2013) 345–353
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Effect of geometry of hydrodynamically cavitating device on degradation of orange-G Virendra Kumar Saharan, Manav A. Rizwani, Aqeel A. Malani, Aniruddha B. Pandit ⇑ Chemical Engineering Department, Institute of Chemical Technology, Mumbai 400019, India
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 7 August 2012 Accepted 10 August 2012 Available online 23 August 2012 Keywords: Degradation of Orange-G Hydrodynamic Cavitation Circular Venturi Slit Venturi Orifice Plate
a b s t r a c t In this research work, we have carried out geometric optimization of different cavitating devices using degradation of orange-G dye [OG] as a model pollutant. Three different cavitating devices viz. orifice plate, circular venturi and slit venturi were optimized and the degradation of orange-G dye was studied. The optimization of all three cavitating devices was done in terms of fluid inlet pressure to the cavitating devices and cavitation number. The effect of pH and initial concentration of the dye on the degradation rate was also studied. The geometry of cavitating device (flow cross sectional area, perimeter, shape, etc.) was found to be an important parameter in getting the maximum cavitational effect using hydrodynamic cavitation. The cavitational yield of all three cavitating devices were compared on the basis of mg of total organic carbon (TOC) reduction per unit energy supplied. The slit venturi gives almost 50% higher degradation rate and cavitational yield among all three cavitating devices studied for the same amount of energy supplied. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Water pollution has become a major problem due to large amount of industrial effluent discharged into the water body coming from many chemical processing industry such as textile, pharmaceutical, pesticides and petrochemical. These effluents contain large amounts of organic compounds such as textile dyes, aromatic compounds, chlorinated hydrocarbons, and phenolic compounds. These organic molecules are bio-refractory or toxic to the microorganisms. Hence, conventional biological processes are not able to completely degrade these compounds [1]. Due to increasing awareness about the environment and more stringent environmental regulations, treatment of industrial wastewater has always been a key aspect of research. In the past few years many researchers have tried different methods for the degradation of organic pollutants. These include carbon bed adsorption, biological methods, oxidation using chlorination and ozonation, electrochemical methods, membrane processes and other advanced oxidation techniques [2–5]. In last decade a new technology called as hydrodynamic cavitation (HC) has been extensively studied by many researchers in the area of waste water treatment because this technique is energy efficient and also easy to scale up to industrial scale. The hydrodynamic cavitation technique has been tested for the degradation of various organic pollutants such as pesticides
⇑ Corresponding author. Tel.: +91 22 3361 2012; fax: +91 22 33611020. E-mail address: [email protected] (A.B. Pandit). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.08.011
pharmaceutical drug and textiles dyes by many researchers throughout the world and showing positive results in getting degradation of bio-refractory pollutants [6–21]. In hydrodynamic cavitation, cavities are generated by passing the liquid through a constriction such as throttling valve, orifice plate, venturi etc. When the liquid passes through the constriction, the kinetic energy associated with the liquid increases at an expense of the local pressure. When the pressure at the throat or vena-contracta of the constriction falls below the vapor pressure of the liquid, the liquid flashes, generating number of cavities that subsequently collapse when the pressure recovers downstream of the mechanical constriction [6]. The effects of cavity collapse are in terms of creation of hot spots, releasing highly reactive free radicals, surface cleaning and/or erosion, and enhancement in local transport (heat, mass and momentum) rates. The collapse of bubbles generates localized ‘‘hot spots’’ with transient temperature of the order of 10,000 K, and pressures of about 1000 atm [22]. Under such extreme conditions water molecules are dissociated into OH and H radicals. These OH radicals then diffuse into the bulk liquid medium where they react with organic pollutants and oxidize/ mineralize them. The intense shockwave and collapse pressure pulse produced also has the capability of molecule break-down/ rearrangement facilitating this mineralization process. The efficiency of the hydrodynamic cavitation is very much dependent on the number of cavitational event (number of cavities) occurring inside a cavitating device and the intensity of cavity collapse, which in turn depends on the geometry of the cavitating device and the flow conditions of the liquid i.e., the scale
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of turbulence and the rate of pressure recovery. The optimum cavitational yield of the hydrodynamic cavitation is the result of several optimized parameters such as number of cavitational events occurring inside a cavitating device, residence time of cavity in the low pressure zone and the rate of pressure recovery downstream of the throat [23]. All these parameters needs to be clubbed and optimized together to get the enhanced cavitational yield from hydrodynamic cavitation device because considering only one parameter in the design of a cavitating device would not result in the possible explanation of all cavitational conditions for the desired effects. This is due to the fact that all these parameters are not independent. Thus, the cavitational condition inside a cavitating device can be altered by changing the ratio of the perimeter of cavitating holes to its cross sectional area (a), the ratio of the throat length to its diameter/height and divergent angle (in the case of a venturi). As these parameters affects the number of cavitational events occurring inside a cavitating device, residence time of cavity in low pressure zone and the rate of pressure recovery downstream of the throat. Sivakumar and Pandit [7] have studied the effect of geometry of the multiple hole orifice plates on the degradation of a cationic dye rhodamine B solution. They have concluded that in hydrodynamic cavitation, altering the flow geometry and hence the turbulent pressure fluctuation frequency (fT) could enhance the cavitational yield. Optimum frequency of turbulence can be achieved by manipulating the flow conditions and geometry of the cavitation device. They have observed that for the plates having the same cross sectional flow area, it is advisable to use a plate with a smaller hole size opening, thereby increasing the number of holes in order to achieve a larger area occupied by the shear layer due to higher perimeter value. Because, for smaller hole sizes, the value of fT increases, leading to a more efficient cavity collapse and higher cavitational yield. Hence, the ratio of the perimeter of the throat to its open area (a) is an important parameter which determines the number density of cavities that can be generated. Also, if there is a choice on the magnitude of the flow area, lower percentage of cross sectional flow areas should be chosen, as with a decrease in flow area, the intensity of cavitation increases. Bashir et al. have [23] carried out CFD based optimization of the important geometrical parameters of a cavitating venturi. They have found that the ability of a cavitating device to generate cavities and the overall cavitational yield depends on the several parameters such as ratio of the perimeter of cavitating hole to the cross sectional flow area of its constriction (a), the ratio of the throat length to its height (in the case of a slit venturi) and the divergence angle. There is not much work reported in the literature on the design and optimization of different cavitating device. Most of the studies were focused on the degradation of different pollutants using single or multiple hole orifice plates and synergetic effect of hydrodynamic cavitation and other additives. No studies have been found on the comparison of venturies of different shapes and orifice plate and the subsequent effect of the cavitating device on the degradation kinetics. As discussed earlier the geometry of a cavitating device has a strong influence on the entire cavitation (inception, growth and collapse) behaviors. The CFD analysis by Bashir et al. [23] has also theoretically indicated such a possibility and hence it is worth validating these numerical predictions using experiments. In the present work we have carried out degradation of orange-G dye [OG] and optimization of three different cavitating device viz. a single hole orifice plate, circular venturi and a slit venturi. The cavitational yield of all three cavitating devices were compared on the basis of amount of TOC reduced per unit energy supplied.
OH N
N SO 3Na
SO 3Na Fig. 1. Molecular structure of orange-G.
2. Materials and methods 2.1. Materials Orange-G dye (molecular weight: 452.38 g/mol; molecular formula: C16H10N2Na2O7S2) was purchased from S.D. fine chemicals (India). The chemical structure of orange-G dye is shown in Fig. 1. All the solutions were prepared with tap water as a dissolution medium. The pH of the solution was maintained using H2SO4. 2.2. Hydrodynamic cavitation reactor The experimental setup is shown in Fig. 2. The setup includes a holding tank of 15 l volume, a positive displacement pump of power rating 1.1 kW, control valves (V1, V2, and V3), and flanges to accommodate the cavitating device in the main line and a bypass line to control the flow through the main line. The suction side of the pump is connected to the bottom of the tank and discharge from the pump branches into two lines; the main line and a bypass line. The main line consists of a flange which houses the cavitating device which can be either orifice or a venturi. The main line flow rate was adjusted by changing the number of piston strokes per unit time of the pump, which affects the total flow generated. Additionally, a valve is also provided in the bypass line to control the liquid flow through the main line. Both the mainline and bypass line terminate well inside the tank below the liquid level to avoid any induction of air into the liquid due to the falling liquid jet. Fig. 3 shows three cavitating devices used in this work. The dimensions of circular and slit venturi are given in Table 1 and a orifice plate (1 mm thickness) with 2 mm hole at the center is shown in Fig. 3. As explained earlier (Section 1) in the case of venturi the important parameters which needs to be considered are throat area, the ratio of the throat length to its diameter/height and divergent angle. Bashir et al. [23] have explained the effect of all these parameter on the cavity dynamic and cavitational yield of a cavitating device using CFD study and proposed optimized cavitating device for best cavitational activity. The dimensions of
Cooling Bypass water out line
Main line P2 Cavitating device
Cooling water in
P1 Tank
V1
V2
V3
Pump
P1, P2 - Pressure gauges V1,V2,V3 - Control valves
Fig. 2. Schematic representation of hydrodynamic cavitation reactor set-up.
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(b) Slit venturi
(a) Circular venturi
(c) Orifice plate Fig. 3. Schematic diagram of cavitating devices.
Table 1 Dimension of circular and slit venturi. Dimension
Circular venturi
Slit venturi
Dimension of throat
Circular hole of 2 mm diameter
Venturi length Length of convergent section Length of divergent section Half angle of convergent section Half angle of divergent section
87 mm 18 mm 67 mm 22.6° 6.4°
W = 6.0 mm; H = 1.9 mm; L = 1.9 mm 87 mm 20 mm 65 mm 23.5° 5.5°
and for a constant circulation time of 2 h. The concentration of OG was varied from 30 to 150 lM for the study of the degradation kinetics. The pressure study was done over a range of 2–7 bar. The pressure shown here are gauge pressure in bar. The temperature of the solution during experiments was kept constant in all the cases at about 32 ± 2 °C and was maintained by circulating cooling water through the jacket provided to the holding tank. The absorbance of OG dye was monitored using UV-Spectrophotometer (Shimadzu1800) and then the concentration of OG was calculated by analyzing the absorbance at the wavelength of 478.5 nm. The absorbance spectrum of orange-G dye is shown in the Fig. 4. It is observed from the spectrum that the absorbance of orange-G dye is reducing with increasing treatment time through the hydrodynamic cavitation device. The complete mineralization was analyzed by measuring the total organic carbon (TOC) content of the dye solution using TOC analyzer (ANATOC II, SGE International Pty Ltd., Australia). 3. Result and discussion 3.1. Hydraulic characteristics The hydraulic characteristics of the all the cavitating devices have been studied first by measuring the main line flow rate at different pump discharge pressures (inlet pressure to the cavitating device). The cavitation number (Cv) and power dissipated (PD) into the system per unit power supplied (PI) to the system was then calculated. The power dissipated into the system is defined as Power dissipated into the system ðP D ; J=sÞ ¼ Pressure drop across the cavitating device ðDP; PaÞ Volumetric flow rate through the cavitating device ðV o ; m3 =sÞ
Fig. 4. Absorbance spectra of orange-G dye w.r.t. treatment time (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: orifice plate).
cavitating devices used in our study are based on the optimized parameter found out by Bashir et al..
The Cavitation number is a dimensionless number used to characterize the condition and degree of cavitation in hydraulic devices [24–25]. The cavitation number is defined as
CV ¼ 2.3. Experimental and Analytical Methods Hydrodynamic cavitation based degradation of OG dye was carried out at different conditions using fixed solution volume of 6 l
p2 pv 1 2
qv 2o
! ð1Þ
Where, p2 is the fully recovered downstream pressure, pv is the vapor pressure of the liquid, vo is the velocity at the throat of the cavitating constriction which can be calculated by knowing the
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1400
Cavitation number (Cv)
2.5
1200
Cv (orifice plate) Cv (circular venturi) Cv (slit venturi) VO (circular venturi) VO (orifice plate) VO (slit venturi)
2 1.5
1000 800 600
1
400
0.5 0
200
0
1
2
3
4
5
6
7
8
9
10
11
Volumetric flow rate (VO), LPH
3
0
Inlet pessure (bar) Fig. 5. Effect of inlet pressure on the main line flow rate (VO) and Cavitation Number (CV).
in the liquid velocity. Yan and Thorpe [26] have reported the cavitation number for the inception of cavity for different orifice sizes. They observed that for a given size orifice, the cavitation inception number remains constant within an experimental error for a specified liquid. The cavitation inception number does not change with the liquid velocity and is a constant for a given orifice size and is found to be increasing with an increasing size and dimension of the orifice. Fig. 6a and 6b shows the power dissipated (DP volumetric flow rate) into the system per unit power supplied (1.1 kW in all the cases) to the system at different inlet pressures and operating cavitation number. It can be observed that power dissipated into the system per unit power supplied to the system is higher for the slit venturi for a given inlet pressure and cavitation number as compared to orifice plate and circular venturi. The power dissipated into the system for the slit venturi is almost three times higher than circular venturi and 4 times higher than orifice plate.
main line flow rate and area of the opening. A sample calculation for the cavitation number and power dissipation is shown in Appendix A. Under ideal condition cavities are generated at a condition Cv 6 1 but in many cases cavities are known to get generated at a value of Cv greater than one due to the presence of some dissolved gases and suspended particles which provide additional nuclei for the cavities to form [24]. The cavitation number at which first cavity appears is called cavity inception number (Cvi). Fig. 5 shows the effect of the pump discharge pressure (inlet pressure to the cavitating device) on the main line flow rate and cavitation number. The liquid flow rate through the main line increases with an increase in the pump discharge pressure (inlet pressure to the cavitating device). It was found that cavitation number decreases with an increase in inlet pressure to the cavitating device. An increase in the discharge pressure increases the flow through the main line, the velocity at the throat of the venturi also increases, which subsequently reduces the cavitation number as per the definition of Cv. It was observed that in the case of slit venturi, a higher volumetric flow rate was obtained for a given pressure drop as compared to orifice plate and circular venturi. The cavitation number in both the venturi was less as compared to orifice plate at a given inlet pressure because of higher volumetric flow rate obtained in both the venturies. The number of cavities generated increases with a decrease in cavitation number i.e. with an increase
3.2. Effect of initial dye concentration In order to investigate the kinetics of degradation, experiments were conducted with different initial concentration ranging from 30 to 150 lM. The method of initial rate was used to determine the reaction order and the specific rate constant. The operating pressure and the pH of solution were kept constant in all the
0.14 0.12
PD / PI
0.1 0.08 0.06 circular venturi
0.04
orifice plate slit venturi
0.02 0
0
1
2
3
4
5
6
7
8
9
10
Inlet pressure (bar) Fig. 6a. Effect of inlet pressure on the power dissipation in a cavitating device.
11
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0.14 0.12
circular venturi
PD / PI
0.1
orifice plate slit venturi
0.08 0.06 0.04 0.02 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Cavitation number (CV) Fig. 6b. Effect of cavitation number on the power dissipation in a cavitating device.
experiments at 5 bar and 2.0 respectively and hydrodynamic cavitation was carried out using circular venturi. Initial rates were calculated at different initial dye concentration. In the case of degradation of dye where the concentration of dye is very small (<150 lM) the rate can be expressed as follows: n
RA ¼ kC Ao
ð2Þ
lnðRA Þ ¼ lnðkÞ þ n lnðC Ao Þ
ð3Þ
Where, RA is the initial rate of degradation of orange-G in mol l1 min1, CAo is the initial concentration of orange-G in mol lit1, k is the rate constant (litn 1moln 1min1) and n is the order of reaction. Fig 7a shows the plot of ln(RA) v/s ln(CAo), the slope of which gives the order of reaction. From the Fig. 7a it is clear that the degradation of orange-G follows first order kinetics and also the plot of rate v/s concentration (Fig 7b) is a straight line passing through the origin, which also confirms that the degradation of orange-G is a first order reaction. The rate of degradation increases with an increase in initial concentration of dye and the first order rate constant calculated was found to be constant, irrespective of initial dye concentration, confirming the validity of the kinetic expression.
3.3. Effect of pH Solution pH is an important parameter in determining the efficiency of hydrodynamic cavitation as it affects the chemical property of the solution and the possible location of the solute (at the interface or in bulk). Many researchers have studied the effect of solution pH on the efficacy of cavitation process in degradation of organic pollutant [27–29]. In this study, the effect of pH was investigated by carrying out HC experiments (using circular venturi as a cavitating device) at different pH in the range 2–13. Fig. 8 shows the effect of pH on the decolorisation rate of OG. The results indicate that the rate of decolorisation increases with a decrease in solution pH i.e. acidic medium (lower pH) is more favorable for the degradation of OG using HC. Much lower decolorisation rate was observed at pH 9.0. About 75.72% decolorisation was obtained at pH 2.0 using HC with circular venturi as a cavitating device. However no decolorisation was observed at pH 11.0 and 13.0. The two main mechanisms for the degradation of pollutants using hydrodynamic cavitation are the thermal decomposition/ pyrolysis of the pollutant molecules entrapped inside the cavity and near to the cavity surface during the collapse of the cavity and secondly, the reaction of OH radicals with the pollutant occurring at the cavity-water interface and in the bulk medium. In the
ln(CAO) -11
-10.5
-10
-9.5
-9
-8.5
-8
-10 -10.5
-11.5 -12
ln(RA)
-11 y = 1.004x - 3.5089 R² = 0.986
-12.5 -13 -13.5 -14 -14.5 Fig. 7a. Degradation kinetic of OG (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: circular venturi).
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45 40
RA x 107 (mol/lit/min)
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
CAO (µM) Fig. 7b. Effect of initial concentration on degradation rate (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: circular venturi).
case of non volatile pollutant the main mechanism for the degradation of pollutants will be the attack of hydroxyl radicals on the pollutant molecules at the cavity-water interface and in the bulk fluid medium. The mechanical effects are also significant. In some cases the intensity of shockwaves generated by the collapsing cavity can break molecular bonds, especially the complex large molecular weight compounds. Therefore the orientation of the pollutants molecule in a solution (especially near or at the cavitywater interface) is very important in getting the maximum effect. The orientation of pollutant molecule is very much dependent on the state of the molecule, whether molecular or ionic. The enhancement in the decolorisation/degradation rate at lower pH can be attributed to the fact that dye molecule is present in the molecular state at lower pH, hence can easily enter the region of gas–water interface of cavities due to hydrophobic nature and thus, is more readily subjected to the OH radical attack and also to the thermal decomposition. Thus, the overall decomposition of OG is attributed to the pyrolysis and free radical attack occurred at both the cavity-water interface and in the bulk liquid medium. Whereas in the basic medium the dye molecules gets ionized and becomes hydrophilic in nature thereby remains in the bulk liquid. In the alkaline medium, the ionic species of OG predominated and therefore it cannot evaporate into the gaseous region or enter the region of the cavity-water interface leaving the decomposition
35
K x 103 (1/min)
30 25 20 15 10 5 0 2
3
4
6
7.3
9
pH of solution Fig. 8. Effect of solution pH on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, inlet pressure: 5 bar, cavitating device: circular venturi).
of OG to occur only in the bulk liquid medium by the attack of OH radicals, and therefore reducing the decolorisation rate. Ku et al. [27] have studied the effect of pH on the degradation of 2-chlorophenol using ultrasound and showed that the 2chlorophenol decomposes faster at pH 3 as compared to pH 11. They have stated that the decomposition is rapid in acidic solutions where molecular form of 2-chlorophenol dominates, part of the molecular species may evaporate into the gaseous region, therefore the overall decomposition of 2-chlorophenol in acidic solutions is considered to take place in both the gaseous and film regions. For alkaline solutions, the non-evaporative ionic species of 2chlorophenol is the predominant species and the decomposition of 2-chlorophenol is assumed to occur only in the film and bulk region. 3.4. Optimization of cavitating devices (effect of inlet pressure and cavitation number) The inlet pressures to the cavitating device and cavitation number are the two important parameters which affects the cavitational condition inside a cavitating device. The number of cavities being generated and the cavitational intensity (collapse pressure magnitude) depends very much on the inlet pressure and cavitation number. In this study, the optimization of three cavitating devices was carried out by studying the decolorisation rate of OG at different inlet pressure and cavitation number. The optimized condition for a cavitating device is the inlet pressure and cavitation number at which maximum decolorisation rate is achieved. Experiments were carried out at different inlet pressure ranging from 3 to 7 bar. The initial concentration of dye and pH was adjusted at 50 lM and 2.0 respectively. Fig. 9 shows the effect of inlet pressure and cavitation number on the decolorisation rate of OG. It has been observed that the decolorisation rate increases with an increase in the inlet pressure reaching a maximum and then drops for all the cavitating devices studied in this work. It can be observed that there is an optimum inlet pressure and cavitation number at which decolorisation rate is maximum for all three cavitating device. The optimized inlet pressure is 3 bar (CV = 0.29) for the slit venturi and 5 bar for the circular venturi (CV = 0.15) and orifice plate (CV = 0.24) respectively. The maximum decolorisation rate was obtained in the case of slit venturi, whereas lowest decolorisation rate was obtained in the case of orifice plate. About 92% decolorisation was obtained in the case of slit venturi (almost 88% decolorisation takes place in the first 45 min of circulation through the cavitating device), whereas 76% and 45% decolorisation was obtained with circular venturi and orifice plate respectively in 2 h at optimized
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60 CV = 0.29
circular venturi
K x 103 (1/min)
50
slit venturi orifice plate
CV = 0.22
40 CV = 0.45
CV = 0.15
30 CV = 0.18
CV = 0.13
CV = 0.21
20 10
CV = 0.38
CV = 0.29
CV = 0.24
CV = 0.21
CV = 0.11 CV = 0.18
0 1
2
3
4
5
6
7
8
Inlet pressure (bar) Fig. 9. Effect of inlet pressure and cavitation number (CV) on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, pH of solution: 2.0).
inlet pressure of all three cavitating devices. It was also observed that slit venturi gives higher % decolorisation as compared to circular venturi and orifice plate for the same number of passes through the cavitating devices. Fig. 10 shows the change in % decolorisation with number of passes for three different cavitating devices at their optimized inlet pressure. Where, the number of passes can be defined as follows.
Number of passes ¼ ðVolumetric flow rate ðVo Þ=Total volume of solution in the holding tankÞ Time of operation As the pressure increases, main line flow rate housing cavitating device increases, so the velocity at the throat of cavitating device increases, which subsequently reduces the cavitation number as per the definition of Cv. As the cavitation number decreases more number of cavities are formed which results into higher cavitational yield, hence higher decolorisation rate was obtained at lower cavitation number. The quantum of the total collapse pressure is the result of collapse pressure due to a single cavity and number of cavities being generated and thus the cavitational intensity due to cavity collapse will be higher with more cavities. On the other hand at a very low cavitation number (higher inlet pressure) the number density of cavity becomes so high that the condition of choked cavitation occurs. Once the cavitation device is completely filled with a large number of cavities (choked cavitation) these
cavities start coalescing to form a larger cavitational bubble (cavity cloud). These larger bubbles escape the liquid without collapsing or result into an incomplete and/or cushioned collapse, thus reducing the cavitational yield and thereby reducing the decolorisation rate after the optimum reached in all the cavitating devices. Saharan et al. [8] have reported the degradation of reactive red 120 dye using hydrodynamic cavitation with circular venturi (same as used in this study) as a cavitating device and found out that degradation rate increased with an increase in the inlet pressure reaching a maximum (5 bar) and then reduced. They have stated that the reduction in degradation rate beyond 5 bar inlet pressure can be attributed to the condition of choked cavitation (CV 6 0.15). To validate the hypothesis of choked condition they have carried out photographic study to analyze the cavity behavior inside a circular venturi. They have studied how the cavity formation occurs inside a transparent circular venturi with an increase in the inlet pressure. They observed that initially at lower pressure the number density of cavities are low and these cavities behave as individual cavities and they collapsed as soon as they come out on the downstream of the venturi. No cavity cloud was observed up to 5 bar inlet pressure (Cv = 0.15) as till this value of the inlet pressure the volume fraction occupied by the cavity is quite low and each cavity tends to behave individually. At the operating condition of inlet pressure of higher than 5 bar, number density of cavities becomes so high that entire downstream area is
100 90
% Decolorisation
80 70 60
circular venturi at 5 bar inet pressure
50 40
slit venturi at 3 bar inlet pressure
30 20
orifice plate at 5 bar inlet pressure
10 0 0
50
100
150
200
250
300
350
400
Number of passes Fig. 10. Effect of number of passes on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, pH of solution: 2.0).
mg of TOC reduced/ Energy supplied
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6.00E-06 5.00E-06 4.00E-06 3.00E-06 2.00E-06 1.00E-06 0.00E+00
slit venturi
circular venturi
orifice plate
Fig. 11. Cavitational yield of different cavitating devices at optimized conditions (operating pressures: slit venturi, 3 bar; circular venturi, 5 bar; orifice plate, 5 bar).
filled with cavities and these cavities then start coalescing with each other and form a cavity cloud. The first clear cavity cloud was observed at 6 bar pressure (Cv = 0.13) and with further increase in inlet pressure almost entire downstream area is filled with cavity cloud. SenthilKumar et al. [30] have also reported the decomposition of aqueous KI solution using hydrodynamic cavitation. They have shown that iodine liberation increased with a decrease in cavitation number, reaches a maxima and then drops. The optimum cavitation number found in their work was again in the range of 0.15 to 0.25 for all the orifice plates studied in their work, which is similar to the optimum reported in this work.
3.5. Comparison of cavitating devices in terms of energy efficiency The energy efficiency of three cavitating devices was compared on the basis of cavitational yield. The cavitational yield is defined as the total quantum (mg) of TOC reduced per unit energy supplied. The calculation for the cavitational yield is shown in Appendix A. The experiments were conducted at optimized condition (inlet pressure and cavitation number) of different cavitating devices and TOC was measured. The concentration of OG and the pH of the solution were kept constant in all the experiments at 50 lM and 2.0 respectively. Fig. 11 shows the cavitational yield for different cavitating devices. It has been observed the slit venturi gives higher cavitational yield as compared to circular venturi and orifice plate. As discussed in the previous section the decolorisation rate and the % decolorisation per pass is also higher in the case of slit venturi as compared to other two cavitating device (Fig. 9 and 10). About 37% reduction in TOC was obtained with slit venturi, whereas 28 and 14% reduction in TOC was obtained with circular venturi and orifice plate respectively in 2 h. As discussed earlier (Section 3.1) in the case of slit venturi, power dissipated into the system is higher as compared to circular venturi and orifice plate for all the inlet pressure and resulting cavitation numbers. The higher dissipated power in the case of slit venturi enhances the number of cavitational events occurring inside the cavitating device and hence the cavitational yield. As discussed earlier, the final collapse pressure and the intensity of cavity collapse and hence the effect of HC on degradation of organic pollutants depends on the number of cavities being generated, the maximum size attained by a cavity before its collapse and the rate of pressure recovery. In the case of HC, all these parameters depends on the geometry of the cavitating device such as throat size, the ratio of throat perimeter to its cross sectional flow area and the angle of divergent section in the case of venturi. In case of both the venturies the pressure recovers smoothly due to
divergent angle and cavities get enough time to grow to maximum size before collapsing, whereas in the case of orifice plate the pressure recover immediately and cavities collapses before attaining a maximum size, thereby reducing the intensity of cavity collapse or cavitational yield for the orifice plate. In the case of venturies the cavitational zone spreads to most of the divergence section as compared to orifice plate. Therefore pollutant molecule will experience cavitational condition for a longer time in the case of both the venturies as compared to orifice plate and hence higher cavitation yield (more reduction in color and TOC) is obtained in the case of both the venturies as compared to orifice plate. Bashir et al. have [23] carried out CFD based optimization of the important geometrical parameters of a cavitating venturi (circular and slit venturi). They have found that the ratio of the perimeter of the venturi to the cross sectional flow area of its constriction quantifies the number of cavities being generated. The ratio of the throat length to its height (in the case of a slit venturi) controls the maximum size of the cavity and the angle of the divergence section controls the rate of collapse of a cavity. Based on the numerical study, they have concluded that a slit venturi (a = 2.7) with the slit length equal to its height (1:1) and a half angle of divergence section of 5.5 degrees is an optimum geometry for best cavitational activity. They have observed that the length of cavitational zone and the number of cavitational events is higher in the case of slit venturi as compared to the circular venturi and hence the maximum cavitational yield is expected to occur in the case of slit venturi. The higher cavitational yield obtained in our study for the slit venturi as compared to circular venturi and orifice plate thus, is consistent with the CFD result found out by Bashir et al. 2011. Hence, it can be concluded that throat geometry, throat size and divergent angles are the important parameters which affects the cavitational condition inside a cavitating device and hence should be optimized to obtain highest cavitational yield.
4. Conclusions The efficiency of the hydrodynamic cavitation found to be dependent on the geometry of the cavitating device and operating parameters (inlet pressure and cavitation number). By manipulating the operating conditions such as inlet pressure and cavitation number, the intensity of cavitation and hence the chemical effect associated with it can be controlled. The number of cavitational event occurring inside a cavitating device and intensity of cavity collapse found to be dependent on the geometry of cavitating device. The study indicates that the maximum cavitational yield through the cavitating device can only be obtained by considering the effect of all these parameters collectively. By considering only one parameter in the design of a cavitating device would not result in the possible explanation of all cavitational conditions for the desired effects. The study reveals that the effects of the physical and chemical properties of the solution are also significant. The pH of the solution and state of the molecule whether ionic or molecular plays an important role in getting the maximum degradation rate using hydrodynamic cavitation as it affects the orientation of the pollutant molecule in the solution. The CFD based simulation as a qualitative design tool also has a future in the designing of the optimized hydrodynamic cavitating device.
Acknowledgement Mr. Virendra Kumar Saharan would like to thank DIISR, Australia and DST, GOI for providing financial support under the India-Australia Strategic Research Funding Program.
V.K. Saharan et al. / Ultrasonics Sonochemistry 20 (2013) 345–353
Appendix A Sample calculation of cavitation number and cavitational yield for circular venturi at optimized condition of 5 bar inlet pressure. Inlet fluid pressure = 601325 Pa. Downstream pressure (p2) = 101325 Pa. Vapor pressure of water at 30 °C (pv) = 4242.14 Pa.
Volumetric flow rate at5 bar pressure ðV o Þ ¼ 410 LPH ¼ 1:14 104 m3 =s 2
Area of flow ðao Þ ¼ ðp=4Þ do ðm2 Þ where do is the diameter of the throat.
Area of flow ðao Þ ¼ ðp=4Þ ð2 103 Þ2 ¼ 3:14 106 m2 Velocity at the throat ðvo Þ ¼ V o =ao ðm=sÞ ¼ ð1:14 104 Þ=ð3:14 106 Þ ¼ 36:30 m=s Cavitation numberðC V Þ ¼ ðP 2 P v Þ=ð1=2qv 2o Þ ¼ ð101325 4242:14Þ=ð0:5 1000 ð36:3Þ2 Þ ¼ 0:15
Number of passes ¼ ðVolumetric flow rate ðV o Þ=Total volume of solution in the holding tankÞ Time of operation Number of passes in 15 min ¼ ðð1:14 104 Þ=ð6 103 ÞÞ ð15 60Þ 17 Power dissipated into the system ðPD Þ ¼ DP V o ðJ=sÞ ¼ 5 105 1:14 104 ¼ 57 J=s Power input to the system ðPI Þ ¼ 1:1 kW ¼ 1100 J=s Power dissipated into the system=Power input to the system ðP D =P I Þ ¼ 57=1100 ¼ 0:051
Amount of TOC reduced in2 h ¼ mg of TOC reduced ðmg=lÞ volume of solution ðlÞ ¼ 5 6 ¼ 30 mg Energy input to the system in2 h ¼ 1100ðJ=sÞ ð2 3600ÞðsÞ ¼ 7:92 106 J Cavitational yield ¼ mg of TOC reduced=energy supplied Cavitational yield ¼ 30=7:92 106 ¼ 3:78 106 References [1] V.K. Saharan, A.B. Pandit, P.S. SatishKumar, S. Anandan, Hydrodynamic cavitation as an advanced oxidation technique for the degradation of Acid Red 88 dye, Ind. Eng. Chem. Res. 51 (2012) 1981–1989.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Effect of geometry of hydrodynamically cavitating device on degradation of orange-G Virendra Kumar Saharan, Manav A. Rizwani, Aqeel A. Malani, Aniruddha B. Pandit ⇑ Chemical Engineering Department, Institute of Chemical Technology, Mumbai 400019, India
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 7 August 2012 Accepted 10 August 2012 Available online 23 August 2012 Keywords: Degradation of Orange-G Hydrodynamic Cavitation Circular Venturi Slit Venturi Orifice Plate
a b s t r a c t In this research work, we have carried out geometric optimization of different cavitating devices using degradation of orange-G dye [OG] as a model pollutant. Three different cavitating devices viz. orifice plate, circular venturi and slit venturi were optimized and the degradation of orange-G dye was studied. The optimization of all three cavitating devices was done in terms of fluid inlet pressure to the cavitating devices and cavitation number. The effect of pH and initial concentration of the dye on the degradation rate was also studied. The geometry of cavitating device (flow cross sectional area, perimeter, shape, etc.) was found to be an important parameter in getting the maximum cavitational effect using hydrodynamic cavitation. The cavitational yield of all three cavitating devices were compared on the basis of mg of total organic carbon (TOC) reduction per unit energy supplied. The slit venturi gives almost 50% higher degradation rate and cavitational yield among all three cavitating devices studied for the same amount of energy supplied. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Water pollution has become a major problem due to large amount of industrial effluent discharged into the water body coming from many chemical processing industry such as textile, pharmaceutical, pesticides and petrochemical. These effluents contain large amounts of organic compounds such as textile dyes, aromatic compounds, chlorinated hydrocarbons, and phenolic compounds. These organic molecules are bio-refractory or toxic to the microorganisms. Hence, conventional biological processes are not able to completely degrade these compounds [1]. Due to increasing awareness about the environment and more stringent environmental regulations, treatment of industrial wastewater has always been a key aspect of research. In the past few years many researchers have tried different methods for the degradation of organic pollutants. These include carbon bed adsorption, biological methods, oxidation using chlorination and ozonation, electrochemical methods, membrane processes and other advanced oxidation techniques [2–5]. In last decade a new technology called as hydrodynamic cavitation (HC) has been extensively studied by many researchers in the area of waste water treatment because this technique is energy efficient and also easy to scale up to industrial scale. The hydrodynamic cavitation technique has been tested for the degradation of various organic pollutants such as pesticides
⇑ Corresponding author. Tel.: +91 22 3361 2012; fax: +91 22 33611020. E-mail address: [email protected] (A.B. Pandit). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.08.011
pharmaceutical drug and textiles dyes by many researchers throughout the world and showing positive results in getting degradation of bio-refractory pollutants [6–21]. In hydrodynamic cavitation, cavities are generated by passing the liquid through a constriction such as throttling valve, orifice plate, venturi etc. When the liquid passes through the constriction, the kinetic energy associated with the liquid increases at an expense of the local pressure. When the pressure at the throat or vena-contracta of the constriction falls below the vapor pressure of the liquid, the liquid flashes, generating number of cavities that subsequently collapse when the pressure recovers downstream of the mechanical constriction [6]. The effects of cavity collapse are in terms of creation of hot spots, releasing highly reactive free radicals, surface cleaning and/or erosion, and enhancement in local transport (heat, mass and momentum) rates. The collapse of bubbles generates localized ‘‘hot spots’’ with transient temperature of the order of 10,000 K, and pressures of about 1000 atm [22]. Under such extreme conditions water molecules are dissociated into OH and H radicals. These OH radicals then diffuse into the bulk liquid medium where they react with organic pollutants and oxidize/ mineralize them. The intense shockwave and collapse pressure pulse produced also has the capability of molecule break-down/ rearrangement facilitating this mineralization process. The efficiency of the hydrodynamic cavitation is very much dependent on the number of cavitational event (number of cavities) occurring inside a cavitating device and the intensity of cavity collapse, which in turn depends on the geometry of the cavitating device and the flow conditions of the liquid i.e., the scale
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of turbulence and the rate of pressure recovery. The optimum cavitational yield of the hydrodynamic cavitation is the result of several optimized parameters such as number of cavitational events occurring inside a cavitating device, residence time of cavity in the low pressure zone and the rate of pressure recovery downstream of the throat [23]. All these parameters needs to be clubbed and optimized together to get the enhanced cavitational yield from hydrodynamic cavitation device because considering only one parameter in the design of a cavitating device would not result in the possible explanation of all cavitational conditions for the desired effects. This is due to the fact that all these parameters are not independent. Thus, the cavitational condition inside a cavitating device can be altered by changing the ratio of the perimeter of cavitating holes to its cross sectional area (a), the ratio of the throat length to its diameter/height and divergent angle (in the case of a venturi). As these parameters affects the number of cavitational events occurring inside a cavitating device, residence time of cavity in low pressure zone and the rate of pressure recovery downstream of the throat. Sivakumar and Pandit [7] have studied the effect of geometry of the multiple hole orifice plates on the degradation of a cationic dye rhodamine B solution. They have concluded that in hydrodynamic cavitation, altering the flow geometry and hence the turbulent pressure fluctuation frequency (fT) could enhance the cavitational yield. Optimum frequency of turbulence can be achieved by manipulating the flow conditions and geometry of the cavitation device. They have observed that for the plates having the same cross sectional flow area, it is advisable to use a plate with a smaller hole size opening, thereby increasing the number of holes in order to achieve a larger area occupied by the shear layer due to higher perimeter value. Because, for smaller hole sizes, the value of fT increases, leading to a more efficient cavity collapse and higher cavitational yield. Hence, the ratio of the perimeter of the throat to its open area (a) is an important parameter which determines the number density of cavities that can be generated. Also, if there is a choice on the magnitude of the flow area, lower percentage of cross sectional flow areas should be chosen, as with a decrease in flow area, the intensity of cavitation increases. Bashir et al. have [23] carried out CFD based optimization of the important geometrical parameters of a cavitating venturi. They have found that the ability of a cavitating device to generate cavities and the overall cavitational yield depends on the several parameters such as ratio of the perimeter of cavitating hole to the cross sectional flow area of its constriction (a), the ratio of the throat length to its height (in the case of a slit venturi) and the divergence angle. There is not much work reported in the literature on the design and optimization of different cavitating device. Most of the studies were focused on the degradation of different pollutants using single or multiple hole orifice plates and synergetic effect of hydrodynamic cavitation and other additives. No studies have been found on the comparison of venturies of different shapes and orifice plate and the subsequent effect of the cavitating device on the degradation kinetics. As discussed earlier the geometry of a cavitating device has a strong influence on the entire cavitation (inception, growth and collapse) behaviors. The CFD analysis by Bashir et al. [23] has also theoretically indicated such a possibility and hence it is worth validating these numerical predictions using experiments. In the present work we have carried out degradation of orange-G dye [OG] and optimization of three different cavitating device viz. a single hole orifice plate, circular venturi and a slit venturi. The cavitational yield of all three cavitating devices were compared on the basis of amount of TOC reduced per unit energy supplied.
OH N
N SO 3Na
SO 3Na Fig. 1. Molecular structure of orange-G.
2. Materials and methods 2.1. Materials Orange-G dye (molecular weight: 452.38 g/mol; molecular formula: C16H10N2Na2O7S2) was purchased from S.D. fine chemicals (India). The chemical structure of orange-G dye is shown in Fig. 1. All the solutions were prepared with tap water as a dissolution medium. The pH of the solution was maintained using H2SO4. 2.2. Hydrodynamic cavitation reactor The experimental setup is shown in Fig. 2. The setup includes a holding tank of 15 l volume, a positive displacement pump of power rating 1.1 kW, control valves (V1, V2, and V3), and flanges to accommodate the cavitating device in the main line and a bypass line to control the flow through the main line. The suction side of the pump is connected to the bottom of the tank and discharge from the pump branches into two lines; the main line and a bypass line. The main line consists of a flange which houses the cavitating device which can be either orifice or a venturi. The main line flow rate was adjusted by changing the number of piston strokes per unit time of the pump, which affects the total flow generated. Additionally, a valve is also provided in the bypass line to control the liquid flow through the main line. Both the mainline and bypass line terminate well inside the tank below the liquid level to avoid any induction of air into the liquid due to the falling liquid jet. Fig. 3 shows three cavitating devices used in this work. The dimensions of circular and slit venturi are given in Table 1 and a orifice plate (1 mm thickness) with 2 mm hole at the center is shown in Fig. 3. As explained earlier (Section 1) in the case of venturi the important parameters which needs to be considered are throat area, the ratio of the throat length to its diameter/height and divergent angle. Bashir et al. [23] have explained the effect of all these parameter on the cavity dynamic and cavitational yield of a cavitating device using CFD study and proposed optimized cavitating device for best cavitational activity. The dimensions of
Cooling Bypass water out line
Main line P2 Cavitating device
Cooling water in
P1 Tank
V1
V2
V3
Pump
P1, P2 - Pressure gauges V1,V2,V3 - Control valves
Fig. 2. Schematic representation of hydrodynamic cavitation reactor set-up.
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(b) Slit venturi
(a) Circular venturi
(c) Orifice plate Fig. 3. Schematic diagram of cavitating devices.
Table 1 Dimension of circular and slit venturi. Dimension
Circular venturi
Slit venturi
Dimension of throat
Circular hole of 2 mm diameter
Venturi length Length of convergent section Length of divergent section Half angle of convergent section Half angle of divergent section
87 mm 18 mm 67 mm 22.6° 6.4°
W = 6.0 mm; H = 1.9 mm; L = 1.9 mm 87 mm 20 mm 65 mm 23.5° 5.5°
and for a constant circulation time of 2 h. The concentration of OG was varied from 30 to 150 lM for the study of the degradation kinetics. The pressure study was done over a range of 2–7 bar. The pressure shown here are gauge pressure in bar. The temperature of the solution during experiments was kept constant in all the cases at about 32 ± 2 °C and was maintained by circulating cooling water through the jacket provided to the holding tank. The absorbance of OG dye was monitored using UV-Spectrophotometer (Shimadzu1800) and then the concentration of OG was calculated by analyzing the absorbance at the wavelength of 478.5 nm. The absorbance spectrum of orange-G dye is shown in the Fig. 4. It is observed from the spectrum that the absorbance of orange-G dye is reducing with increasing treatment time through the hydrodynamic cavitation device. The complete mineralization was analyzed by measuring the total organic carbon (TOC) content of the dye solution using TOC analyzer (ANATOC II, SGE International Pty Ltd., Australia). 3. Result and discussion 3.1. Hydraulic characteristics The hydraulic characteristics of the all the cavitating devices have been studied first by measuring the main line flow rate at different pump discharge pressures (inlet pressure to the cavitating device). The cavitation number (Cv) and power dissipated (PD) into the system per unit power supplied (PI) to the system was then calculated. The power dissipated into the system is defined as Power dissipated into the system ðP D ; J=sÞ ¼ Pressure drop across the cavitating device ðDP; PaÞ Volumetric flow rate through the cavitating device ðV o ; m3 =sÞ
Fig. 4. Absorbance spectra of orange-G dye w.r.t. treatment time (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: orifice plate).
cavitating devices used in our study are based on the optimized parameter found out by Bashir et al..
The Cavitation number is a dimensionless number used to characterize the condition and degree of cavitation in hydraulic devices [24–25]. The cavitation number is defined as
CV ¼ 2.3. Experimental and Analytical Methods Hydrodynamic cavitation based degradation of OG dye was carried out at different conditions using fixed solution volume of 6 l
p2 pv 1 2
qv 2o
! ð1Þ
Where, p2 is the fully recovered downstream pressure, pv is the vapor pressure of the liquid, vo is the velocity at the throat of the cavitating constriction which can be calculated by knowing the
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1400
Cavitation number (Cv)
2.5
1200
Cv (orifice plate) Cv (circular venturi) Cv (slit venturi) VO (circular venturi) VO (orifice plate) VO (slit venturi)
2 1.5
1000 800 600
1
400
0.5 0
200
0
1
2
3
4
5
6
7
8
9
10
11
Volumetric flow rate (VO), LPH
3
0
Inlet pessure (bar) Fig. 5. Effect of inlet pressure on the main line flow rate (VO) and Cavitation Number (CV).
in the liquid velocity. Yan and Thorpe [26] have reported the cavitation number for the inception of cavity for different orifice sizes. They observed that for a given size orifice, the cavitation inception number remains constant within an experimental error for a specified liquid. The cavitation inception number does not change with the liquid velocity and is a constant for a given orifice size and is found to be increasing with an increasing size and dimension of the orifice. Fig. 6a and 6b shows the power dissipated (DP volumetric flow rate) into the system per unit power supplied (1.1 kW in all the cases) to the system at different inlet pressures and operating cavitation number. It can be observed that power dissipated into the system per unit power supplied to the system is higher for the slit venturi for a given inlet pressure and cavitation number as compared to orifice plate and circular venturi. The power dissipated into the system for the slit venturi is almost three times higher than circular venturi and 4 times higher than orifice plate.
main line flow rate and area of the opening. A sample calculation for the cavitation number and power dissipation is shown in Appendix A. Under ideal condition cavities are generated at a condition Cv 6 1 but in many cases cavities are known to get generated at a value of Cv greater than one due to the presence of some dissolved gases and suspended particles which provide additional nuclei for the cavities to form [24]. The cavitation number at which first cavity appears is called cavity inception number (Cvi). Fig. 5 shows the effect of the pump discharge pressure (inlet pressure to the cavitating device) on the main line flow rate and cavitation number. The liquid flow rate through the main line increases with an increase in the pump discharge pressure (inlet pressure to the cavitating device). It was found that cavitation number decreases with an increase in inlet pressure to the cavitating device. An increase in the discharge pressure increases the flow through the main line, the velocity at the throat of the venturi also increases, which subsequently reduces the cavitation number as per the definition of Cv. It was observed that in the case of slit venturi, a higher volumetric flow rate was obtained for a given pressure drop as compared to orifice plate and circular venturi. The cavitation number in both the venturi was less as compared to orifice plate at a given inlet pressure because of higher volumetric flow rate obtained in both the venturies. The number of cavities generated increases with a decrease in cavitation number i.e. with an increase
3.2. Effect of initial dye concentration In order to investigate the kinetics of degradation, experiments were conducted with different initial concentration ranging from 30 to 150 lM. The method of initial rate was used to determine the reaction order and the specific rate constant. The operating pressure and the pH of solution were kept constant in all the
0.14 0.12
PD / PI
0.1 0.08 0.06 circular venturi
0.04
orifice plate slit venturi
0.02 0
0
1
2
3
4
5
6
7
8
9
10
Inlet pressure (bar) Fig. 6a. Effect of inlet pressure on the power dissipation in a cavitating device.
11
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0.14 0.12
circular venturi
PD / PI
0.1
orifice plate slit venturi
0.08 0.06 0.04 0.02 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Cavitation number (CV) Fig. 6b. Effect of cavitation number on the power dissipation in a cavitating device.
experiments at 5 bar and 2.0 respectively and hydrodynamic cavitation was carried out using circular venturi. Initial rates were calculated at different initial dye concentration. In the case of degradation of dye where the concentration of dye is very small (<150 lM) the rate can be expressed as follows: n
RA ¼ kC Ao
ð2Þ
lnðRA Þ ¼ lnðkÞ þ n lnðC Ao Þ
ð3Þ
Where, RA is the initial rate of degradation of orange-G in mol l1 min1, CAo is the initial concentration of orange-G in mol lit1, k is the rate constant (litn 1moln 1min1) and n is the order of reaction. Fig 7a shows the plot of ln(RA) v/s ln(CAo), the slope of which gives the order of reaction. From the Fig. 7a it is clear that the degradation of orange-G follows first order kinetics and also the plot of rate v/s concentration (Fig 7b) is a straight line passing through the origin, which also confirms that the degradation of orange-G is a first order reaction. The rate of degradation increases with an increase in initial concentration of dye and the first order rate constant calculated was found to be constant, irrespective of initial dye concentration, confirming the validity of the kinetic expression.
3.3. Effect of pH Solution pH is an important parameter in determining the efficiency of hydrodynamic cavitation as it affects the chemical property of the solution and the possible location of the solute (at the interface or in bulk). Many researchers have studied the effect of solution pH on the efficacy of cavitation process in degradation of organic pollutant [27–29]. In this study, the effect of pH was investigated by carrying out HC experiments (using circular venturi as a cavitating device) at different pH in the range 2–13. Fig. 8 shows the effect of pH on the decolorisation rate of OG. The results indicate that the rate of decolorisation increases with a decrease in solution pH i.e. acidic medium (lower pH) is more favorable for the degradation of OG using HC. Much lower decolorisation rate was observed at pH 9.0. About 75.72% decolorisation was obtained at pH 2.0 using HC with circular venturi as a cavitating device. However no decolorisation was observed at pH 11.0 and 13.0. The two main mechanisms for the degradation of pollutants using hydrodynamic cavitation are the thermal decomposition/ pyrolysis of the pollutant molecules entrapped inside the cavity and near to the cavity surface during the collapse of the cavity and secondly, the reaction of OH radicals with the pollutant occurring at the cavity-water interface and in the bulk medium. In the
ln(CAO) -11
-10.5
-10
-9.5
-9
-8.5
-8
-10 -10.5
-11.5 -12
ln(RA)
-11 y = 1.004x - 3.5089 R² = 0.986
-12.5 -13 -13.5 -14 -14.5 Fig. 7a. Degradation kinetic of OG (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: circular venturi).
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45 40
RA x 107 (mol/lit/min)
35 30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
16
CAO (µM) Fig. 7b. Effect of initial concentration on degradation rate (Conditions: volume of solution: 6 l, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: circular venturi).
case of non volatile pollutant the main mechanism for the degradation of pollutants will be the attack of hydroxyl radicals on the pollutant molecules at the cavity-water interface and in the bulk fluid medium. The mechanical effects are also significant. In some cases the intensity of shockwaves generated by the collapsing cavity can break molecular bonds, especially the complex large molecular weight compounds. Therefore the orientation of the pollutants molecule in a solution (especially near or at the cavitywater interface) is very important in getting the maximum effect. The orientation of pollutant molecule is very much dependent on the state of the molecule, whether molecular or ionic. The enhancement in the decolorisation/degradation rate at lower pH can be attributed to the fact that dye molecule is present in the molecular state at lower pH, hence can easily enter the region of gas–water interface of cavities due to hydrophobic nature and thus, is more readily subjected to the OH radical attack and also to the thermal decomposition. Thus, the overall decomposition of OG is attributed to the pyrolysis and free radical attack occurred at both the cavity-water interface and in the bulk liquid medium. Whereas in the basic medium the dye molecules gets ionized and becomes hydrophilic in nature thereby remains in the bulk liquid. In the alkaline medium, the ionic species of OG predominated and therefore it cannot evaporate into the gaseous region or enter the region of the cavity-water interface leaving the decomposition
35
K x 103 (1/min)
30 25 20 15 10 5 0 2
3
4
6
7.3
9
pH of solution Fig. 8. Effect of solution pH on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, inlet pressure: 5 bar, cavitating device: circular venturi).
of OG to occur only in the bulk liquid medium by the attack of OH radicals, and therefore reducing the decolorisation rate. Ku et al. [27] have studied the effect of pH on the degradation of 2-chlorophenol using ultrasound and showed that the 2chlorophenol decomposes faster at pH 3 as compared to pH 11. They have stated that the decomposition is rapid in acidic solutions where molecular form of 2-chlorophenol dominates, part of the molecular species may evaporate into the gaseous region, therefore the overall decomposition of 2-chlorophenol in acidic solutions is considered to take place in both the gaseous and film regions. For alkaline solutions, the non-evaporative ionic species of 2chlorophenol is the predominant species and the decomposition of 2-chlorophenol is assumed to occur only in the film and bulk region. 3.4. Optimization of cavitating devices (effect of inlet pressure and cavitation number) The inlet pressures to the cavitating device and cavitation number are the two important parameters which affects the cavitational condition inside a cavitating device. The number of cavities being generated and the cavitational intensity (collapse pressure magnitude) depends very much on the inlet pressure and cavitation number. In this study, the optimization of three cavitating devices was carried out by studying the decolorisation rate of OG at different inlet pressure and cavitation number. The optimized condition for a cavitating device is the inlet pressure and cavitation number at which maximum decolorisation rate is achieved. Experiments were carried out at different inlet pressure ranging from 3 to 7 bar. The initial concentration of dye and pH was adjusted at 50 lM and 2.0 respectively. Fig. 9 shows the effect of inlet pressure and cavitation number on the decolorisation rate of OG. It has been observed that the decolorisation rate increases with an increase in the inlet pressure reaching a maximum and then drops for all the cavitating devices studied in this work. It can be observed that there is an optimum inlet pressure and cavitation number at which decolorisation rate is maximum for all three cavitating device. The optimized inlet pressure is 3 bar (CV = 0.29) for the slit venturi and 5 bar for the circular venturi (CV = 0.15) and orifice plate (CV = 0.24) respectively. The maximum decolorisation rate was obtained in the case of slit venturi, whereas lowest decolorisation rate was obtained in the case of orifice plate. About 92% decolorisation was obtained in the case of slit venturi (almost 88% decolorisation takes place in the first 45 min of circulation through the cavitating device), whereas 76% and 45% decolorisation was obtained with circular venturi and orifice plate respectively in 2 h at optimized
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60 CV = 0.29
circular venturi
K x 103 (1/min)
50
slit venturi orifice plate
CV = 0.22
40 CV = 0.45
CV = 0.15
30 CV = 0.18
CV = 0.13
CV = 0.21
20 10
CV = 0.38
CV = 0.29
CV = 0.24
CV = 0.21
CV = 0.11 CV = 0.18
0 1
2
3
4
5
6
7
8
Inlet pressure (bar) Fig. 9. Effect of inlet pressure and cavitation number (CV) on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, pH of solution: 2.0).
inlet pressure of all three cavitating devices. It was also observed that slit venturi gives higher % decolorisation as compared to circular venturi and orifice plate for the same number of passes through the cavitating devices. Fig. 10 shows the change in % decolorisation with number of passes for three different cavitating devices at their optimized inlet pressure. Where, the number of passes can be defined as follows.
Number of passes ¼ ðVolumetric flow rate ðVo Þ=Total volume of solution in the holding tankÞ Time of operation As the pressure increases, main line flow rate housing cavitating device increases, so the velocity at the throat of cavitating device increases, which subsequently reduces the cavitation number as per the definition of Cv. As the cavitation number decreases more number of cavities are formed which results into higher cavitational yield, hence higher decolorisation rate was obtained at lower cavitation number. The quantum of the total collapse pressure is the result of collapse pressure due to a single cavity and number of cavities being generated and thus the cavitational intensity due to cavity collapse will be higher with more cavities. On the other hand at a very low cavitation number (higher inlet pressure) the number density of cavity becomes so high that the condition of choked cavitation occurs. Once the cavitation device is completely filled with a large number of cavities (choked cavitation) these
cavities start coalescing to form a larger cavitational bubble (cavity cloud). These larger bubbles escape the liquid without collapsing or result into an incomplete and/or cushioned collapse, thus reducing the cavitational yield and thereby reducing the decolorisation rate after the optimum reached in all the cavitating devices. Saharan et al. [8] have reported the degradation of reactive red 120 dye using hydrodynamic cavitation with circular venturi (same as used in this study) as a cavitating device and found out that degradation rate increased with an increase in the inlet pressure reaching a maximum (5 bar) and then reduced. They have stated that the reduction in degradation rate beyond 5 bar inlet pressure can be attributed to the condition of choked cavitation (CV 6 0.15). To validate the hypothesis of choked condition they have carried out photographic study to analyze the cavity behavior inside a circular venturi. They have studied how the cavity formation occurs inside a transparent circular venturi with an increase in the inlet pressure. They observed that initially at lower pressure the number density of cavities are low and these cavities behave as individual cavities and they collapsed as soon as they come out on the downstream of the venturi. No cavity cloud was observed up to 5 bar inlet pressure (Cv = 0.15) as till this value of the inlet pressure the volume fraction occupied by the cavity is quite low and each cavity tends to behave individually. At the operating condition of inlet pressure of higher than 5 bar, number density of cavities becomes so high that entire downstream area is
100 90
% Decolorisation
80 70 60
circular venturi at 5 bar inet pressure
50 40
slit venturi at 3 bar inlet pressure
30 20
orifice plate at 5 bar inlet pressure
10 0 0
50
100
150
200
250
300
350
400
Number of passes Fig. 10. Effect of number of passes on decolorisation rate of OG (Conditions: volume of solution: 6 l, initial concentration: 50 lM, pH of solution: 2.0).
mg of TOC reduced/ Energy supplied
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6.00E-06 5.00E-06 4.00E-06 3.00E-06 2.00E-06 1.00E-06 0.00E+00
slit venturi
circular venturi
orifice plate
Fig. 11. Cavitational yield of different cavitating devices at optimized conditions (operating pressures: slit venturi, 3 bar; circular venturi, 5 bar; orifice plate, 5 bar).
filled with cavities and these cavities then start coalescing with each other and form a cavity cloud. The first clear cavity cloud was observed at 6 bar pressure (Cv = 0.13) and with further increase in inlet pressure almost entire downstream area is filled with cavity cloud. SenthilKumar et al. [30] have also reported the decomposition of aqueous KI solution using hydrodynamic cavitation. They have shown that iodine liberation increased with a decrease in cavitation number, reaches a maxima and then drops. The optimum cavitation number found in their work was again in the range of 0.15 to 0.25 for all the orifice plates studied in their work, which is similar to the optimum reported in this work.
3.5. Comparison of cavitating devices in terms of energy efficiency The energy efficiency of three cavitating devices was compared on the basis of cavitational yield. The cavitational yield is defined as the total quantum (mg) of TOC reduced per unit energy supplied. The calculation for the cavitational yield is shown in Appendix A. The experiments were conducted at optimized condition (inlet pressure and cavitation number) of different cavitating devices and TOC was measured. The concentration of OG and the pH of the solution were kept constant in all the experiments at 50 lM and 2.0 respectively. Fig. 11 shows the cavitational yield for different cavitating devices. It has been observed the slit venturi gives higher cavitational yield as compared to circular venturi and orifice plate. As discussed in the previous section the decolorisation rate and the % decolorisation per pass is also higher in the case of slit venturi as compared to other two cavitating device (Fig. 9 and 10). About 37% reduction in TOC was obtained with slit venturi, whereas 28 and 14% reduction in TOC was obtained with circular venturi and orifice plate respectively in 2 h. As discussed earlier (Section 3.1) in the case of slit venturi, power dissipated into the system is higher as compared to circular venturi and orifice plate for all the inlet pressure and resulting cavitation numbers. The higher dissipated power in the case of slit venturi enhances the number of cavitational events occurring inside the cavitating device and hence the cavitational yield. As discussed earlier, the final collapse pressure and the intensity of cavity collapse and hence the effect of HC on degradation of organic pollutants depends on the number of cavities being generated, the maximum size attained by a cavity before its collapse and the rate of pressure recovery. In the case of HC, all these parameters depends on the geometry of the cavitating device such as throat size, the ratio of throat perimeter to its cross sectional flow area and the angle of divergent section in the case of venturi. In case of both the venturies the pressure recovers smoothly due to
divergent angle and cavities get enough time to grow to maximum size before collapsing, whereas in the case of orifice plate the pressure recover immediately and cavities collapses before attaining a maximum size, thereby reducing the intensity of cavity collapse or cavitational yield for the orifice plate. In the case of venturies the cavitational zone spreads to most of the divergence section as compared to orifice plate. Therefore pollutant molecule will experience cavitational condition for a longer time in the case of both the venturies as compared to orifice plate and hence higher cavitation yield (more reduction in color and TOC) is obtained in the case of both the venturies as compared to orifice plate. Bashir et al. have [23] carried out CFD based optimization of the important geometrical parameters of a cavitating venturi (circular and slit venturi). They have found that the ratio of the perimeter of the venturi to the cross sectional flow area of its constriction quantifies the number of cavities being generated. The ratio of the throat length to its height (in the case of a slit venturi) controls the maximum size of the cavity and the angle of the divergence section controls the rate of collapse of a cavity. Based on the numerical study, they have concluded that a slit venturi (a = 2.7) with the slit length equal to its height (1:1) and a half angle of divergence section of 5.5 degrees is an optimum geometry for best cavitational activity. They have observed that the length of cavitational zone and the number of cavitational events is higher in the case of slit venturi as compared to the circular venturi and hence the maximum cavitational yield is expected to occur in the case of slit venturi. The higher cavitational yield obtained in our study for the slit venturi as compared to circular venturi and orifice plate thus, is consistent with the CFD result found out by Bashir et al. 2011. Hence, it can be concluded that throat geometry, throat size and divergent angles are the important parameters which affects the cavitational condition inside a cavitating device and hence should be optimized to obtain highest cavitational yield.
4. Conclusions The efficiency of the hydrodynamic cavitation found to be dependent on the geometry of the cavitating device and operating parameters (inlet pressure and cavitation number). By manipulating the operating conditions such as inlet pressure and cavitation number, the intensity of cavitation and hence the chemical effect associated with it can be controlled. The number of cavitational event occurring inside a cavitating device and intensity of cavity collapse found to be dependent on the geometry of cavitating device. The study indicates that the maximum cavitational yield through the cavitating device can only be obtained by considering the effect of all these parameters collectively. By considering only one parameter in the design of a cavitating device would not result in the possible explanation of all cavitational conditions for the desired effects. The study reveals that the effects of the physical and chemical properties of the solution are also significant. The pH of the solution and state of the molecule whether ionic or molecular plays an important role in getting the maximum degradation rate using hydrodynamic cavitation as it affects the orientation of the pollutant molecule in the solution. The CFD based simulation as a qualitative design tool also has a future in the designing of the optimized hydrodynamic cavitating device.
Acknowledgement Mr. Virendra Kumar Saharan would like to thank DIISR, Australia and DST, GOI for providing financial support under the India-Australia Strategic Research Funding Program.
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Appendix A Sample calculation of cavitation number and cavitational yield for circular venturi at optimized condition of 5 bar inlet pressure. Inlet fluid pressure = 601325 Pa. Downstream pressure (p2) = 101325 Pa. Vapor pressure of water at 30 °C (pv) = 4242.14 Pa.
Volumetric flow rate at5 bar pressure ðV o Þ ¼ 410 LPH ¼ 1:14 104 m3 =s 2
Area of flow ðao Þ ¼ ðp=4Þ do ðm2 Þ where do is the diameter of the throat.
Area of flow ðao Þ ¼ ðp=4Þ ð2 103 Þ2 ¼ 3:14 106 m2 Velocity at the throat ðvo Þ ¼ V o =ao ðm=sÞ ¼ ð1:14 104 Þ=ð3:14 106 Þ ¼ 36:30 m=s Cavitation numberðC V Þ ¼ ðP 2 P v Þ=ð1=2qv 2o Þ ¼ ð101325 4242:14Þ=ð0:5 1000 ð36:3Þ2 Þ ¼ 0:15
Number of passes ¼ ðVolumetric flow rate ðV o Þ=Total volume of solution in the holding tankÞ Time of operation Number of passes in 15 min ¼ ðð1:14 104 Þ=ð6 103 ÞÞ ð15 60Þ 17 Power dissipated into the system ðPD Þ ¼ DP V o ðJ=sÞ ¼ 5 105 1:14 104 ¼ 57 J=s Power input to the system ðPI Þ ¼ 1:1 kW ¼ 1100 J=s Power dissipated into the system=Power input to the system ðP D =P I Þ ¼ 57=1100 ¼ 0:051
Amount of TOC reduced in2 h ¼ mg of TOC reduced ðmg=lÞ volume of solution ðlÞ ¼ 5 6 ¼ 30 mg Energy input to the system in2 h ¼ 1100ðJ=sÞ ð2 3600ÞðsÞ ¼ 7:92 106 J Cavitational yield ¼ mg of TOC reduced=energy supplied Cavitational yield ¼ 30=7:92 106 ¼ 3:78 106 References [1] V.K. Saharan, A.B. Pandit, P.S. SatishKumar, S. Anandan, Hydrodynamic cavitation as an advanced oxidation technique for the degradation of Acid Red 88 dye, Ind. Eng. Chem. Res. 51 (2012) 1981–1989.
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