- Email: [email protected]
Experimental verification of the effect of liquid deposition on droplet size measured in a rectangular Venturi scrubber
Chemical Engineering and Processing 50 (2011) 1137–1142
Contents lists available at SciVerse ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Experimental verification of the effect of liquid deposition on droplet size measured in a rectangular Venturi scrubber V.G. Guerra, J.A.S. Gonc¸alves ∗ , J.R. Coury Federal University of São Carlos, Department of Chemical Engineering, Via Washington Luiz, Km. 235, 13565-905, São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 15 September 2010 Received in revised form 30 March 2011 Accepted 10 September 2011 Available online 29 September 2011 Keywords: Venturi scrubber Droplet size Liquid film Film fraction Transversal injection Jet atomization
a b s t r a c t It has been reported by previous studies [1–7] that droplet size in a Pease-Anthony Venturi scrubber depends on the jet atomization conditions, such as jet velocity and gas velocity. The assumption of this paper is that actual collector-droplet size in a confined tube such as the Venturi scrubber also depends significantly on preferential droplet deposition on the tube walls, which remove preferentially droplets of a certain size from the core, modifying the mean droplet size of the remaining droplets. To account for this effect, the present study is focused on the experimental measurement of the liquid deposition on the walls of a Pease-Anthony Venturi scrubber and the droplet size remaining in the core. The experiments were carried out varying jet penetration and the number of the injection orifices. A correlation, using dimensionless numbers, was proposed to quantify the influence of each experimental condition. The results showed that liquid deposition has a significant influence in actual collector-droplet size inside a Venturi scrubber. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Venturi scrubber is widely used in industry to control particle emission. It is comprised of three principal parts: convergent section, throat and divergent section. A cleaning liquid introduced into the scrubber removes the contaminants from the gas flow. Generally, the liquid is introduced through orifices in the throat of the scrubber as a jet. The high velocity of the gas developed in the throat of the equipment rapidly atomizes the liquid jets into numerous droplets. The droplets act as collectors of the particles contained in the gas and their size and distribution are important parameters for the efficient removal of contaminants and in the energy consumption of the operation [1–8]. Many factors affect the size of the droplets inside the Venturi scrubber, depending on the operating conditions employed, such as: gas velocity, liquid velocity and the number of orifices used to inject the liquid [1]. Operating conditions can, favorably or unfavorably, affect phenomena such as: primary and secondary atomization, coalescence of droplets and the sizes of droplets deposited preferentially on the interior walls of the equipment, among others. During the process of atomization, not all the liquid introduced into the scrubber creates droplets that really act like particle collectors. A fraction of the liquid deposits on the walls of the equipment and flows as a film. The presence of the film affects the equipment’s
∗ Corresponding author. Tel.: +55 16 3351 8045; fax: +55 16 3351 8266. E-mail address: [email protected] (J.A.S. Gonc¸alves). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.09.005
collection performance, since it presents a smaller surface area than the droplets and can increase pressure drop, due to attrition between the gas and the wrinkled surface of the film. The fraction of liquid deposited on the equipment walls is strongly influenced by the distribution of liquid inside it; in turn, distribution is influenced by the penetration of the liquid jet inside the scrubber. Insufficient jet penetrations inside the throat tend to deposit liquid on the walls where the injection orifices are localized and excessive jet penetrations favor liquid deposition on the walls opposite to the injection orifices. Penetration by the jet is directly proportional to the velocity of the liquid and inversely proportional to the velocity of the gas. The empirical correlation developed by Viswanathan et al. [9] is widely used to predict the penetration of the jet into the interior of Venturi scrubbers: ∗∗ = 0.1145
l · Vj g · Vg
do
(1)
in which Vj =
4 · Ql · do2 · No
(2)
According to a study by Wu et al. [10] the rupture of a liquid jet, introduced into an orifice perpendicular to an air stream, happens through the formation of two main regions, as shown in Fig. 1: column breakup and surface breakup of the liquid. The column breakup is located on the upper part of the jet and the surface breakup is located on the lower part of the jet. This suggests that jets with excessive penetration favor the deposition of the largest
1138
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
Fig. 2. Diagram of the Venturi scrubber used in the experimental tests.
Fig. 1. Diagram showing ruptured column and the surface of the liquid jet in a transversal air current [10].
droplets on the wall, the opposite behavior would occur with jets with small penetration. Thus jet penetration also exercises a significant influence on the size of the droplets which deposit on the walls and this will also reflect in the size of the droplets remaining in the core. The size of the droplet collector is a very important parameter that affects the performance of the collection equipment. Nevertheless, there are not many studies developed with the purpose of mathematically finding a correlation to estimate the droplet size in a Venturi scrubber [11,12], this is because of the complexity of the phenomena involved. Quantifying the size of the droplets generated by the atomization process is not a simple task, especially when they are confined inside the throat of a Venturi scrubber. The implications involved in the deposition of part of the droplets created during atomization on the walls of the scrubber or the numbers of orifices for liquid injection are not predicted by the correlations most often used to estimate droplet diameter [11,12]. Due to complex phenomena involved in the processes of atomization, even today computerized models of the process fail due to the great computing effort required to simulate the rupture of a liquid jet and also due to the unsatisfactory development of models to explain formation of the primary spray. Thus, relevant characteristics of the spray, such as characteristic diameter, need to be determined experimentally [13]. The various studies undertaken to determine the size of droplets using atomizers, in general, propose empirical correlations to adjust the diameter obtained experimentally [14–16]. Among the classical correlations used to predict the droplet size in scrubbers, the correlations developed by Nukiyama and Tanasawa [11] and Boll et al. [12] were also obtained empirically and they assume that all liquid injected into Venturi scrubber is atomized and converted into droplets. Eqs. (3) and (4) show the correlation of Boll et al. [12] and Nukiyama and Tanasawa [11], respectively: D32 =
D32 =
4.22 × 10−2 + 5.77 × 10−3 (1000Ql /Qg )
1.932
(3)
Vg1.602 0.585 (Vg − Vd )
+ 1.683 × 10−3 l
l √ l
0.45
1000Ql Qg
On jet injection Venturi scrubbers, however, droplet sizes are most important in the neighborhood of the liquid injection, and, in this region, deposition is much more significant than entrainment. Some of the studies developed on pneumatic atomization use dimensionless numbers raised to powers to quantify the diameter of droplets [13–16,21–25]. Buckingham’s theorem based on dimensional analysis makes possible a strategy to choose relevant data, how it should be presented and which are the parameters that most influence the experimental results. This is a useful technique in all experimental areas of engineering, since if it is possible to identify the important factors involved in a physical process, the dimensional analysis could give form to and find the relationship between them. The purpose of this study is to present experimental data on both droplet size and film fraction in a Venturi scrubber, and to quantitatively assess the influence of different operational variables on the size of the collector droplet in a Venturi scrubber. The important dimensionless numbers were identified and correlated. A correlation to estimate the Sauter diameter can serve as a project tool that evaluates the importance of certain operational parameters and how they influence the size of the droplet inside the Venturi scrubber. 2. Experimental The Venturi scrubber used was constructed in modules and mounted horizontally, having rectangular geometry with a throat height (H) equal to 0.040 m and a width (W) equal to 0.027 m. The orifices for liquid injection, four in total, are 0.001 m in diameter and were located at the beginning of the scrubber’s throat. Water was injected through one to four orifices with the aid of a positive displacement pump. The air stream, responsible for atomizing the liquid, was generated using a blower. A diagram of the Venturi scrubber used in the experimental tests can be seen in Fig. 2. Experimental tests to measure the size of the droplet and extraction of the liquid film were done by varying: gas velocity in the scrubber throat (59–74 m/s), the velocity of the liquid jet (1.6–12.8 m/s) and the number and configuration of the injection orifices. The number of functioning orifices was varied from 1 to 4. Individual orifices could be blocked, allowing for different injection configurations. Five different combinations of injection orifices were used. An overall view of the configuration of the injection of liquid can be seen in Fig. 3 and Table 1.
1.5 (4)
Ambrosini et al. [17], Azzopardi [18], Simmons and Hanratty [19] and Al-Sarkhi and Hanratty [20] presented droplet size correlations for fully developed annular flows. In these correlations, droplet sizes depend strongly on the entrained liquid mass flux.
Table 1 Configurations of liquid injection. Number of active orifices
Orifices in operation
1
1 1 and 3 – mode 1 2 and 4 – mode 2 1, 2 and 3 1 to 4
2 3 4
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
1139
Fig. 3. Diagram of liquid injection in the scrubber throat. Fig. 5. Detail of the window to permit laser analysis of droplets.
The film was extracted from the scrubber walls using the slot method, which was first proposed by Hay et al. [26]. A test module, containing slots for film removal, was connected to the end of the Venturi scrubber’s throat. A diagram of the test section can be seen in Fig. 4. In this piece the liquid film was deflected by a lamina (0.001 m) in the direction of a gap, accumulated in a small reservoir, and flowed onto the outside of the scrubber through a pipe and a hose connected to a restriction. The restriction keeps excessive air from escaping with the film, thus avoiding droplets being carried along with it. The liquid film leaves the equipment due to the difference between the inside and outside (atmospheric) pressure. The liquid mass collected on each of the walls of the scrubber was weighed and summed in order to determine the total fraction of film deposited on the walls. Droplet size in the throat of the scrubber was measured using Malvern Spraytech equipment, which uses laser ray diffraction technique that makes possible in situ measurements. To obtain
visual access to the flow inside of the scrubber, a test module was used. It was connected after the throat. The illustrative diagram of the test section can be seen in Fig. 5. This module consists of quartz windows that allow the passage of the laser, laminas and slots to remove the liquid film and a compressed air injection system, which keeps the droplets from reaching the quartz windows. The test section for measuring droplet size and film fraction was positioned at 0.12 m from the point of injection of liquid in the throat of the equipment. The tests were done at an ambient temperature of 25 ◦ C. 3. Results and discussion The data on droplet size measured experimentally were correlated with operational and geometrical parameters by making use of dimensionless numbers. To apply the dimensional analysis to the process of atomization, the relevant physical factors should first be established. According to Bayvel and Orzechnowsy [27] the parameters that are important to atomization are the following: the diameter of the droplet generated in the atomization process (D32 ), orifice diameter (do ), gas and jet velocities (Vg , Vj ) and the properties of the fluids involved in the process, in this case the surface tension of the liquid (), liquid and gas density (l , g ) and liquid and gas viscosity (l , g ). All these parameters can be described by at most three magnitudes: mass, dimension and time. In this study, in addition to the parameters suggested by Bayvel and Orzechnowsy [27], Sauter’s diameter was also established as a function of Dinj , a representative liquid injection throat diameter, and hydraulic diameter (Dh ), both of these representing the diameter for a rectangular scrubber. The value of Dinj for a rectangular scrubber was defined as observed in Eq. (6). The value of Dinj varies with the injection system, depending on which walls the injection orifices are located: Dinj =
mH + nW m+n
(6)
in which H is the height of the scrubber (0.040 m) and W is the width of the scrubber throat (0.027 mm), as shown in Fig. 3, m is the number of active orifices on the “H-walls”, n is the number of orifices on the “W-walls”. Thus, Sauter’s diameter was established as a function of the following properties: Fig. 4. Test section containing the slot for film removal.
D32 = f (do , Vj , l , l , , Dh , Dinj , Vg , g , g )
(5)
1140
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
+15 %
200
a
0.4
1 orifice; Vg = 59 m/s 2 orifices mode 1; Vg = 59 m/s 2 orifices mode 2; Vg = 59 m/s 3 orifices; Vg = 59 m/s 4 orfices; Vg = 59 m/s 1 orifices; Vg = 69 m/s 2 orifices mode 1; Vg = 69 m/s 2 orifices mode 2; Vg = 69 m/s 3 orifices; Vg = 69 m/s 4 orifices; Vg = 69 m/s
150
Ff(-)
D32experimental (μm)
0.3
-15 % 0.2
100 0.1
1 orifice 2 orifices mode 1 2 orifices mode 2 3 orifices 4 orifices
50
0.0 0.0
50
100
150
0.2
0.3
0.4
0.5
0.6
0.7
0.8
**/Dh (-)
b
0 0
0.1
0.4
200
D32estimated (μm)
1 orifice; Vg 64 m/s 2 orifices mode 1; Vg 2 orifices mode 2; Vg 3 orifices; Vg 64 m/s 4 orfices; Vg 64 m/s 1 orifice; Vg 74 m/s 2 orifices mode 1; Vg 2 orifices mode 2; Vg 3 orifices; Vg 74 m/s 4 orifices; Vg 74 m/s
0.3
By performing an analysis of the important parameters to determining droplet size under different experimental conditions studied and applying Buckingham’s theorem, the following dimensionless numbers were ascertained: D32 =f do
Rel , Weg , Reg ,
Dinj
(7)
Dh
in which Rel = Reynolds number of the liquid (Rel = do · Vj · l /l ), Reg = Reynolds number of the gas (Reg = dh · Vg · g /g ) and Weg = Weber number (Weg = do · Vg2 · g /). Thus the equation for estimating droplet size is: D32 = 1.05 · Rel−0.26 · Weg−1.00 · Reg0.36 · do
Dinj Dh
−0.51
Ff(-)
Fig. 6. Performance of the correlation proposed by Eq. (8). 0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
64 m/s 64 m/s
74 m/s 74 m/s
0.8
**/Dh (-) Fig. 7. (a) Fraction of liquid in the form of a film in function of the jet penetration fraction (**/Dh ) for gas velocities 59 m/s and 69 m/s. (b) Fraction of liquid in the form of a film in function of the jet penetration fraction (**/Dh ) for gas velocities of 64 m/s and 74 m/s.
(8)
The minimal square method was used to determine the adjusted parameters in Eq. (8). Fig. 6 shows the performance of the proposed correlation in Eq. (8) with the Sauter diameters measured in m. Using Fig. 6 one can see that the correlation proposed by Eq. (8) is not entirely satisfactory, since many of the diameter values for the droplets were outside the ±15% range. As can be seen in Fig. 6, the injection configurations that show the largest deviations were for the smallest droplet sizes, which were obtained using the injection configurations for 1 orifice and 2 orifices – mode 1. The greatest jet penetrations occur under these liquid injection conditions. Even if the correlation does not adjust satisfactorily all the experimental data, one can assess some trends that it predicts. It can be verified that droplet diameter is inversely proportional to Rel , which indicates that the increase in liquid flow rate makes the droplet size smaller. This would be in agreement with the rupture breakup proposal Wu et al. [10], wherein the more penetrating jets, i.e., those with higher liquid jet velocity, generate smaller size droplets coming from the lower part of the jet. Nevertheless, the effect of gas velocity on reducing the size of the droplets proves more significant, compared to the influence of the liquid jet velocity. This can be seen by the fact that evaluating the exponents of the number of Weg and Reg , gas velocity would be elevated to −1.64 while jet velocity would raised to −0.26. The Weber number raised to a negative power is physically sensible as droplet diameters tend to get smaller as the liquid surface tension gets smaller. This behavior agrees with that observed in pneumatic atomizers [14,15,25,28]. Through Eq. (8), it can also be verified that the configuration of the liquid injection system contributes in a significant way to
the size of the droplets in the Venturi scrubber, suggesting that preferential droplet deposition maybe an important factor affecting droplet size in Venturi scrubbers. When Dinj is small or Dh is large, jet penetration is more likely to be not excessive, diminishing the deposition of the big droplets on the opposite wall. Thus, the actual collector droplet mean diameter would be bigger. On the other hand, when Dinj /Dh , is large, preferential droplet deposition may remove the big droplets from the core, with the overall effect of diminishing droplet Sauter mean diameter. The parameter Dinj /Dh is only geometrical, it must be investigated whether a more direct parameter such as film fraction could account better for the influence of preferential droplet deposition on the remaining droplet size. In a Pease-Anthony Venturi scrubber, the film is formed after the jet atomization, and deposition is much more important than entrainment during the first stages of film formation. The preferential deposition of a range of droplets would have a decisive influence on the final average size of the droplets that act as contaminant collectors. Thus, in a Venturi scrubber, the parameters that control jet atomization and droplet formation are not the only ones affecting actual droplet size. Fig. 7(a) and (b) shows film fraction (Ff ) as a function of jet penetration (**/Dh ) under different experimental conditions. The deposition of liquid on the throat of the equipment proved to be influenced significantly by the penetration of the jets. It can be observed that for the higher jet penetration fractions the highest film fractions were measured. Under these conditions, the droplets measured in the throat of the scrubber were smaller. This suggests that, jets with excessive penetrations, in addition to favoring improvement in the atomization process, by generating a greater number of smaller droplets, can be causing the preferential deposit of larger droplets on the walls opposite to the injection of
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
1 orifice; Vg = 59 m/s 2 orifices mode 1; Vg = 59 m/s 2 orifices mode 2; Vg = 59 m/s 3 orifices; Vg = 59 m/s 4 orfices; Vg = 59 m/s 1 orifices; Vg = 69 m/s 2 orifices mode 1; Vg = 69 m/s 2 orifices mode 2; Vg = 69 m/s 3 orifices; Vg = 69 m/s 4 orifices; Vg = 69 m/s
0.20 0.18 0.16
D32/do (-)
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1 orifice; Vg = 64 m/s 2 orifices mode 1; Vg = 64 m/s 2 orifices mode 2; Vg = 64 m/s 3 orifices; Vg = 64 m/s 4 orfices; Vg = 64 m/s 1 orifice; Vg = 74 m/s 2 orifices mode 1; Vg = 74 m/s 2 orifices mode 2; Vg = 74 m/s 3 orifices; Vg = 74 m/s 4 orifices; Vg = 74 m/s
0.16
D32/do (-)
0.14 0.12 0.10 0.08 0.06 0.04 0.02
0.00 0.2
0.3
0.4
0.5
0.6
0.7
0.8
**/Dh (-) Fig. 8. (a) Sauter diameter in function of jet the penetration fraction (**/Dh ), for gas velocity of 59 m/s and 64 m/s. (b) Sauter diameter in function of jet the penetration fraction (**/Dh ) for gas velocities of 64 m/s and 74 m/s.
the liquid. This deposition would be caused by the greater inertia of these droplets and favored by its proximity to the walls. The opposite occurs with less penetrating jets. The larger droplets tend to maintain their initial trajectory, due to inertia, while the small droplets are more easily carried by the gas turbulence, easily changing their initial trajectories. In the case of low jet penetration fractions, the larger droplets could flow freely on the air current without hitting the opposite wall since according to the atomization process described by Wu et al. [10], the larger droplets would form on the upper part of the jet. Through Figs. 7(a) and (b) and 8(a) and (b) one can also verify that the situations of larger deposits of liquid film, when the liquid was injected through 1 orifice and 2 orifices – mode 1, the size of the droplets flowing through the throat of the scrubber were smaller, suggesting the existence of preferential deposition. In an attempt to better quantify the influence that the deposition of droplets has on the remaining droplet size, the influence of the fraction of film measured experimentally was included in Eq. (8). In this way, the value Fe , which represents the quantity of liquid that is flowing in the form of droplets in the scrubber throat, was included in Eq. (8). Thus, the equation for estimating the size of the droplet considering the influence of the deposition of droplets on the scrubber walls would take the following form: D32 = 1.05 · Rel−0.21 · Weg−1.32 · Reg0.46 · do in which Fe = (1 − Ff ).
1 orifice 2 orifices mode 1 2 orifices mode 2 3 orifices 4 orifices
50
0
0.18
0.1
100
0
0.20
0.0
-15 %
150
0.8
**/Dh (-)
b
+15 %
200
D32experimental ( m)
a
1141
Dinj Dh
−0.51 · Fe2.63
(9)
50
100
150
200
D32estimated ( m) Fig. 9. Performance of the correlation proposed by Eq. (9).
Fig. 9 shows the comparison between the predicted size given by Eq. (9), to which was added the influence of the liquid film and the experimental results. Fig. 9 shows that the data were better adjusted by Eq. (9), and the droplet size estimated by the equation stayed within the range of ±15%. Using Eq. (9), it can be seen that the constants referring to the influence of the liquid injection configuration were equal to those adjusted in Eq. (8). This suggests that with an added term to include the film deposition effects, behavior with respect to position of the orifice on the throat was the same. The influence of the jet velocity on the reduction of droplet size was diminished while the effect of gas increased. This behavior can be seen in the value of the exponent referring to jet velocity, which went from −0.26 in Eq. (8) to −0.21 in Eq. (9). The influence of gas velocity increased from −1.64 to −2.18. These variations are a result of including the effect of deposition of liquid film on the walls of the equipment. Through the values of the adjusted parameters in Eq. (9), it can be observed that the film fraction deposited exercises a significant influence on droplet size as measured inside the scrubber. The value Fe raised to the highest power (2.63) and the improvement in the adjustment to the experimental data when comparing Fig. 6 with Fig. 9, means that preferential deposition of droplets of a certain size is of significant importance and must be considered in the prediction of the mean diameter of the droplets remaining in the core. These droplets, which flow along with the air current, are responsible for collecting the contaminants present in the gaseous current. 4. Conclusion Through the experimental results and the adjusted correlation for the estimate of droplet size, it can be concluded that: • Under the conditions in which the largest film fractions occur, the smallest size droplets also occur, suggesting the preferential deposition of larger-size droplets on the equipment walls. • The increase in jet velocity, as well as gas velocity contributed to the reduction of droplet size. • The configuration of the liquid injection proved to have a significant influence on the size of droplets. • The correlation proposed (Eq. (9)), in which the effect of the fraction of film that deposits on the wall was added, adjusted the experimental results satisfactorily.
1142
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
• Factors that are not usually taken into consideration in available correlations for the prediction of droplet size in Venturi scrubbers, such as the film fraction and the geometrical details of the liquid injection system, proved to be highly relevant to the final droplet size. Acknowledgements The authors are grateful to CNPq and FAPESP for financial support given to this work. Appendix A. Nomenclature
do D32 Dh Dinj Fe Ff Qg ** Ql No Vd Vg Vj Rel Reg Weg l l g
Diameter of orifice (m) Sauter mean diameter (m) Hydraulic diameter (m) Representative liquid injection throat diameter for a rectangular Venturi scrubber (m) Fraction of liquid flowing in the core of the Venturi throat (dimensionless) Film fraction (dimensionless) Volumetric gas flow rate (m3 /s) Liquid jet central penetration (m) Volumetric liquid flow rate (m3 /s) Number of orifices for liquid injection (dimensionless) Droplet velocity at atomization point (m/s) Gas velocity (m/s) Jet velocity (m/s) Reynolds number of liquid Reynolds number of gas Weber number Liquid viscosity (kg/m s) Liquid density (kg/m3 ) Gas density (kg/m3 ) Surface tension (N/m)
References [1] V.G. Guerra, J.A.S. Gonc¸alves, J.R. Coury, Experimental investigation on the effect of liquid injection by multiple orifices in the formation of droplets in a Venturi scrubber, J. Hazard. Mater. 161 (2009) 351–359. [2] J.A.S. Gonc¸alves, M.A.M. Costa, P.R. Henrique, J.R. Coury, Atomization of liquids in a Pease-Anthony Venturi scrubber. Part I. Jet dynamics, J. Hazard. Mater. 97 (2003) 267–279.
[3] J.A.S. Gonc¸alves, M.A.M. Costa, M.L. Aguiar, J.R. Coury, Atomization of liquids in a Pease-Anthony Venturi scrubber. Part II. Droplet dispersion, J. Hazard. Mater. 116 (2004) 147–157. [4] J.A.S. Gonc¸alves, D. Fernández Alonso, M.A.M. Costa, B.J. Azzopardi, J.R. Coury, Evaluation of the models available for the prediction of pressure drop in Venturi scrubbers, J. Hazard. Mater. 81 (2001) 123–140. [5] D.F. Alonso, J.A. Gonc¸alves, B.J. Azzopardi, J.R. Coury, Drop size measurements in Venturi scrubbers, Chem. Eng. Sci. 56 (2001) 4901–4911. [6] M.A.M. Costa, P.R. Henrique, J.A.S. Gonc¸alves, J.R. Coury, Droplet size in a rectangular Venturi scrubber, Braz. J. Chem. Eng. 21 (2004) 335–343. [7] A.M. Silva, J.C.F. Teixeira, S.F.C.F. Teixeira, Experiments in large scale Venturi scrubber. Part II. Droplet size, Chem. Eng. Process. 48 (2009) 424–431. [8] A.M. Silva, J.C.F. Teixeira, S.F.C.F. Teixeira, Experiments in large scale Venturi scrubber. Part I. Pressure drop, Chem. Eng. Process. 48 (2009) 59–67. [9] S. Viswanathan, C.C.S.T. Pierre, A.W. Gnyp, Jet penetration measurements in a Venturi scrubber, Can. J. Chem. Eng. 61 (1983) 504–508. [10] P-K. Wu, K.A. Kirkendall, R. Fuller, A.S. Nejad, Breakup processes of liquid jets in subsonic crossflows, J. Propul. Power 13 (1997) 64–73. [11] S. Nukiyama, Y. Tanasawa, Experiment on atomization of liquid by means of air stream, Trans. Soc. Mech. Eng. Jpn. 4 (1938) 86–93. [12] R.H. Boll, L.R. Flais, P.W. Maurer, W.L. Thompson, Mean drop size in a full scale Venturi scrubber via transmissometer, J. Air Pollut. Control Assoc. 24 (1974) 934–938. [13] A. Tratining, G. Brenn, T. Strixner, P. Fankhauser, N. Laubacher, M. Stranzinger, Characterization of spray formation from emulsions by pressure-swirl atomizers for spray drying, J. Food Eng. 95 (2009) 126–134. [14] A.H. Lefebvre, Atomization and Sprays, Hemisphere Publishing Corporation, New York, 1989. [15] E.J. Lavernia, Y. Wu, Spray Atomization and Deposition, John Wiley & Sons, Chichester, 1996. [16] H.F. Liu, W.F. Li, X. Gong, X.K. Cao, J.L. Xu, X.L. Chen, Y.F. Wang, G.S. Yu, F.C. Wang, Z.H. Yu, Effect of liquid jet diameter on performance of coaxial two-fluid airblast atomizers, Chem. Eng. Process. 45 (2006) 240–245. [17] W. Ambrosini, P. Andreussi, B.J. Azzopardi, A physically based correlation for drop size in annular flow, Int. J. Multiphase Flow 17 (1991) 497–507. [18] B.J. Azzopardi, Drops in annular two-phase flow, Int. J. Multiphase Flow 23 (1997) S1–S53. [19] M.J.H. Simmons, T.J. Hanratty, Droplet size measurements in horizontal annular gas–liquid flows, Int. J. Multiphase Flow 27 (2001) 861–883. [20] A. Al-Sarki, T.J. Hanratty, Effect of pipe diameter on the drop size in a horizontal annular gás–liquid flow, Int. J. Multiphase Flow 28 (2002) 1617–1629. [21] V. Cleary, P. Bowen, H. Witlox, Flashing liquid jets and two-phase droplet dispersion. I. Experiments for derivation of droplet atomization correlations, J. Hazard. Mater. 142 (2007) 786–796. [22] K. Krishna, T.K. Kim, K.D. Kihm, W.J. Rogers, M.S. Mannan, Predictive correlations for leaking heat transfer fluid aerosols in air, J. Loss Prevent. Proc. 16 (2003) 1–8. [23] K.D. Kihm, G.M. Lyn, S.Y. Son, Atomization of cross-injecting sprays into convective air stream, Atomization Spray 5 (1995) 417–433. [24] B.K. Park, J.S. Lee, K.D. Kihm, Comparative study of twin-fluid atomization using sonic or supersonic gas jets, Atomization Spray 6 (1996) 285–304. [25] K.D. Kihm, N. Chigier, Effect of shock waves on liquid atomization of a twodimensional airblast atomizer, Atomization Spray 1 (1995) 113–136. [26] K.J. Hay, Z-C. Liu, T.J. Hanratty, Relation of deposition to drop size when the rate is nonlinear, Int. J. Multiphase Flow 22 (1996) 829–848. [27] L. Bayvel, Z. Orzechnowsy, Liquid Atomization, Taylor & Francis, Washington, D.C., 1993. [28] G.G. Nasr, A.J. Yule, L. Bendig, Industrial Sprays and Atomization, SpringerVerlag, London, 2002.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Experimental verification of the effect of liquid deposition on droplet size measured in a rectangular Venturi scrubber V.G. Guerra, J.A.S. Gonc¸alves ∗ , J.R. Coury Federal University of São Carlos, Department of Chemical Engineering, Via Washington Luiz, Km. 235, 13565-905, São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 15 September 2010 Received in revised form 30 March 2011 Accepted 10 September 2011 Available online 29 September 2011 Keywords: Venturi scrubber Droplet size Liquid film Film fraction Transversal injection Jet atomization
a b s t r a c t It has been reported by previous studies [1–7] that droplet size in a Pease-Anthony Venturi scrubber depends on the jet atomization conditions, such as jet velocity and gas velocity. The assumption of this paper is that actual collector-droplet size in a confined tube such as the Venturi scrubber also depends significantly on preferential droplet deposition on the tube walls, which remove preferentially droplets of a certain size from the core, modifying the mean droplet size of the remaining droplets. To account for this effect, the present study is focused on the experimental measurement of the liquid deposition on the walls of a Pease-Anthony Venturi scrubber and the droplet size remaining in the core. The experiments were carried out varying jet penetration and the number of the injection orifices. A correlation, using dimensionless numbers, was proposed to quantify the influence of each experimental condition. The results showed that liquid deposition has a significant influence in actual collector-droplet size inside a Venturi scrubber. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Venturi scrubber is widely used in industry to control particle emission. It is comprised of three principal parts: convergent section, throat and divergent section. A cleaning liquid introduced into the scrubber removes the contaminants from the gas flow. Generally, the liquid is introduced through orifices in the throat of the scrubber as a jet. The high velocity of the gas developed in the throat of the equipment rapidly atomizes the liquid jets into numerous droplets. The droplets act as collectors of the particles contained in the gas and their size and distribution are important parameters for the efficient removal of contaminants and in the energy consumption of the operation [1–8]. Many factors affect the size of the droplets inside the Venturi scrubber, depending on the operating conditions employed, such as: gas velocity, liquid velocity and the number of orifices used to inject the liquid [1]. Operating conditions can, favorably or unfavorably, affect phenomena such as: primary and secondary atomization, coalescence of droplets and the sizes of droplets deposited preferentially on the interior walls of the equipment, among others. During the process of atomization, not all the liquid introduced into the scrubber creates droplets that really act like particle collectors. A fraction of the liquid deposits on the walls of the equipment and flows as a film. The presence of the film affects the equipment’s
∗ Corresponding author. Tel.: +55 16 3351 8045; fax: +55 16 3351 8266. E-mail address: [email protected] (J.A.S. Gonc¸alves). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.09.005
collection performance, since it presents a smaller surface area than the droplets and can increase pressure drop, due to attrition between the gas and the wrinkled surface of the film. The fraction of liquid deposited on the equipment walls is strongly influenced by the distribution of liquid inside it; in turn, distribution is influenced by the penetration of the liquid jet inside the scrubber. Insufficient jet penetrations inside the throat tend to deposit liquid on the walls where the injection orifices are localized and excessive jet penetrations favor liquid deposition on the walls opposite to the injection orifices. Penetration by the jet is directly proportional to the velocity of the liquid and inversely proportional to the velocity of the gas. The empirical correlation developed by Viswanathan et al. [9] is widely used to predict the penetration of the jet into the interior of Venturi scrubbers: ∗∗ = 0.1145
l · Vj g · Vg
do
(1)
in which Vj =
4 · Ql · do2 · No
(2)
According to a study by Wu et al. [10] the rupture of a liquid jet, introduced into an orifice perpendicular to an air stream, happens through the formation of two main regions, as shown in Fig. 1: column breakup and surface breakup of the liquid. The column breakup is located on the upper part of the jet and the surface breakup is located on the lower part of the jet. This suggests that jets with excessive penetration favor the deposition of the largest
1138
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
Fig. 2. Diagram of the Venturi scrubber used in the experimental tests.
Fig. 1. Diagram showing ruptured column and the surface of the liquid jet in a transversal air current [10].
droplets on the wall, the opposite behavior would occur with jets with small penetration. Thus jet penetration also exercises a significant influence on the size of the droplets which deposit on the walls and this will also reflect in the size of the droplets remaining in the core. The size of the droplet collector is a very important parameter that affects the performance of the collection equipment. Nevertheless, there are not many studies developed with the purpose of mathematically finding a correlation to estimate the droplet size in a Venturi scrubber [11,12], this is because of the complexity of the phenomena involved. Quantifying the size of the droplets generated by the atomization process is not a simple task, especially when they are confined inside the throat of a Venturi scrubber. The implications involved in the deposition of part of the droplets created during atomization on the walls of the scrubber or the numbers of orifices for liquid injection are not predicted by the correlations most often used to estimate droplet diameter [11,12]. Due to complex phenomena involved in the processes of atomization, even today computerized models of the process fail due to the great computing effort required to simulate the rupture of a liquid jet and also due to the unsatisfactory development of models to explain formation of the primary spray. Thus, relevant characteristics of the spray, such as characteristic diameter, need to be determined experimentally [13]. The various studies undertaken to determine the size of droplets using atomizers, in general, propose empirical correlations to adjust the diameter obtained experimentally [14–16]. Among the classical correlations used to predict the droplet size in scrubbers, the correlations developed by Nukiyama and Tanasawa [11] and Boll et al. [12] were also obtained empirically and they assume that all liquid injected into Venturi scrubber is atomized and converted into droplets. Eqs. (3) and (4) show the correlation of Boll et al. [12] and Nukiyama and Tanasawa [11], respectively: D32 =
D32 =
4.22 × 10−2 + 5.77 × 10−3 (1000Ql /Qg )
1.932
(3)
Vg1.602 0.585 (Vg − Vd )
+ 1.683 × 10−3 l
l √ l
0.45
1000Ql Qg
On jet injection Venturi scrubbers, however, droplet sizes are most important in the neighborhood of the liquid injection, and, in this region, deposition is much more significant than entrainment. Some of the studies developed on pneumatic atomization use dimensionless numbers raised to powers to quantify the diameter of droplets [13–16,21–25]. Buckingham’s theorem based on dimensional analysis makes possible a strategy to choose relevant data, how it should be presented and which are the parameters that most influence the experimental results. This is a useful technique in all experimental areas of engineering, since if it is possible to identify the important factors involved in a physical process, the dimensional analysis could give form to and find the relationship between them. The purpose of this study is to present experimental data on both droplet size and film fraction in a Venturi scrubber, and to quantitatively assess the influence of different operational variables on the size of the collector droplet in a Venturi scrubber. The important dimensionless numbers were identified and correlated. A correlation to estimate the Sauter diameter can serve as a project tool that evaluates the importance of certain operational parameters and how they influence the size of the droplet inside the Venturi scrubber. 2. Experimental The Venturi scrubber used was constructed in modules and mounted horizontally, having rectangular geometry with a throat height (H) equal to 0.040 m and a width (W) equal to 0.027 m. The orifices for liquid injection, four in total, are 0.001 m in diameter and were located at the beginning of the scrubber’s throat. Water was injected through one to four orifices with the aid of a positive displacement pump. The air stream, responsible for atomizing the liquid, was generated using a blower. A diagram of the Venturi scrubber used in the experimental tests can be seen in Fig. 2. Experimental tests to measure the size of the droplet and extraction of the liquid film were done by varying: gas velocity in the scrubber throat (59–74 m/s), the velocity of the liquid jet (1.6–12.8 m/s) and the number and configuration of the injection orifices. The number of functioning orifices was varied from 1 to 4. Individual orifices could be blocked, allowing for different injection configurations. Five different combinations of injection orifices were used. An overall view of the configuration of the injection of liquid can be seen in Fig. 3 and Table 1.
1.5 (4)
Ambrosini et al. [17], Azzopardi [18], Simmons and Hanratty [19] and Al-Sarkhi and Hanratty [20] presented droplet size correlations for fully developed annular flows. In these correlations, droplet sizes depend strongly on the entrained liquid mass flux.
Table 1 Configurations of liquid injection. Number of active orifices
Orifices in operation
1
1 1 and 3 – mode 1 2 and 4 – mode 2 1, 2 and 3 1 to 4
2 3 4
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
1139
Fig. 3. Diagram of liquid injection in the scrubber throat. Fig. 5. Detail of the window to permit laser analysis of droplets.
The film was extracted from the scrubber walls using the slot method, which was first proposed by Hay et al. [26]. A test module, containing slots for film removal, was connected to the end of the Venturi scrubber’s throat. A diagram of the test section can be seen in Fig. 4. In this piece the liquid film was deflected by a lamina (0.001 m) in the direction of a gap, accumulated in a small reservoir, and flowed onto the outside of the scrubber through a pipe and a hose connected to a restriction. The restriction keeps excessive air from escaping with the film, thus avoiding droplets being carried along with it. The liquid film leaves the equipment due to the difference between the inside and outside (atmospheric) pressure. The liquid mass collected on each of the walls of the scrubber was weighed and summed in order to determine the total fraction of film deposited on the walls. Droplet size in the throat of the scrubber was measured using Malvern Spraytech equipment, which uses laser ray diffraction technique that makes possible in situ measurements. To obtain
visual access to the flow inside of the scrubber, a test module was used. It was connected after the throat. The illustrative diagram of the test section can be seen in Fig. 5. This module consists of quartz windows that allow the passage of the laser, laminas and slots to remove the liquid film and a compressed air injection system, which keeps the droplets from reaching the quartz windows. The test section for measuring droplet size and film fraction was positioned at 0.12 m from the point of injection of liquid in the throat of the equipment. The tests were done at an ambient temperature of 25 ◦ C. 3. Results and discussion The data on droplet size measured experimentally were correlated with operational and geometrical parameters by making use of dimensionless numbers. To apply the dimensional analysis to the process of atomization, the relevant physical factors should first be established. According to Bayvel and Orzechnowsy [27] the parameters that are important to atomization are the following: the diameter of the droplet generated in the atomization process (D32 ), orifice diameter (do ), gas and jet velocities (Vg , Vj ) and the properties of the fluids involved in the process, in this case the surface tension of the liquid (), liquid and gas density (l , g ) and liquid and gas viscosity (l , g ). All these parameters can be described by at most three magnitudes: mass, dimension and time. In this study, in addition to the parameters suggested by Bayvel and Orzechnowsy [27], Sauter’s diameter was also established as a function of Dinj , a representative liquid injection throat diameter, and hydraulic diameter (Dh ), both of these representing the diameter for a rectangular scrubber. The value of Dinj for a rectangular scrubber was defined as observed in Eq. (6). The value of Dinj varies with the injection system, depending on which walls the injection orifices are located: Dinj =
mH + nW m+n
(6)
in which H is the height of the scrubber (0.040 m) and W is the width of the scrubber throat (0.027 mm), as shown in Fig. 3, m is the number of active orifices on the “H-walls”, n is the number of orifices on the “W-walls”. Thus, Sauter’s diameter was established as a function of the following properties: Fig. 4. Test section containing the slot for film removal.
D32 = f (do , Vj , l , l , , Dh , Dinj , Vg , g , g )
(5)
1140
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
+15 %
200
a
0.4
1 orifice; Vg = 59 m/s 2 orifices mode 1; Vg = 59 m/s 2 orifices mode 2; Vg = 59 m/s 3 orifices; Vg = 59 m/s 4 orfices; Vg = 59 m/s 1 orifices; Vg = 69 m/s 2 orifices mode 1; Vg = 69 m/s 2 orifices mode 2; Vg = 69 m/s 3 orifices; Vg = 69 m/s 4 orifices; Vg = 69 m/s
150
Ff(-)
D32experimental (μm)
0.3
-15 % 0.2
100 0.1
1 orifice 2 orifices mode 1 2 orifices mode 2 3 orifices 4 orifices
50
0.0 0.0
50
100
150
0.2
0.3
0.4
0.5
0.6
0.7
0.8
**/Dh (-)
b
0 0
0.1
0.4
200
D32estimated (μm)
1 orifice; Vg 64 m/s 2 orifices mode 1; Vg 2 orifices mode 2; Vg 3 orifices; Vg 64 m/s 4 orfices; Vg 64 m/s 1 orifice; Vg 74 m/s 2 orifices mode 1; Vg 2 orifices mode 2; Vg 3 orifices; Vg 74 m/s 4 orifices; Vg 74 m/s
0.3
By performing an analysis of the important parameters to determining droplet size under different experimental conditions studied and applying Buckingham’s theorem, the following dimensionless numbers were ascertained: D32 =f do
Rel , Weg , Reg ,
Dinj
(7)
Dh
in which Rel = Reynolds number of the liquid (Rel = do · Vj · l /l ), Reg = Reynolds number of the gas (Reg = dh · Vg · g /g ) and Weg = Weber number (Weg = do · Vg2 · g /). Thus the equation for estimating droplet size is: D32 = 1.05 · Rel−0.26 · Weg−1.00 · Reg0.36 · do
Dinj Dh
−0.51
Ff(-)
Fig. 6. Performance of the correlation proposed by Eq. (8). 0.2
0.1
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
64 m/s 64 m/s
74 m/s 74 m/s
0.8
**/Dh (-) Fig. 7. (a) Fraction of liquid in the form of a film in function of the jet penetration fraction (**/Dh ) for gas velocities 59 m/s and 69 m/s. (b) Fraction of liquid in the form of a film in function of the jet penetration fraction (**/Dh ) for gas velocities of 64 m/s and 74 m/s.
(8)
The minimal square method was used to determine the adjusted parameters in Eq. (8). Fig. 6 shows the performance of the proposed correlation in Eq. (8) with the Sauter diameters measured in m. Using Fig. 6 one can see that the correlation proposed by Eq. (8) is not entirely satisfactory, since many of the diameter values for the droplets were outside the ±15% range. As can be seen in Fig. 6, the injection configurations that show the largest deviations were for the smallest droplet sizes, which were obtained using the injection configurations for 1 orifice and 2 orifices – mode 1. The greatest jet penetrations occur under these liquid injection conditions. Even if the correlation does not adjust satisfactorily all the experimental data, one can assess some trends that it predicts. It can be verified that droplet diameter is inversely proportional to Rel , which indicates that the increase in liquid flow rate makes the droplet size smaller. This would be in agreement with the rupture breakup proposal Wu et al. [10], wherein the more penetrating jets, i.e., those with higher liquid jet velocity, generate smaller size droplets coming from the lower part of the jet. Nevertheless, the effect of gas velocity on reducing the size of the droplets proves more significant, compared to the influence of the liquid jet velocity. This can be seen by the fact that evaluating the exponents of the number of Weg and Reg , gas velocity would be elevated to −1.64 while jet velocity would raised to −0.26. The Weber number raised to a negative power is physically sensible as droplet diameters tend to get smaller as the liquid surface tension gets smaller. This behavior agrees with that observed in pneumatic atomizers [14,15,25,28]. Through Eq. (8), it can also be verified that the configuration of the liquid injection system contributes in a significant way to
the size of the droplets in the Venturi scrubber, suggesting that preferential droplet deposition maybe an important factor affecting droplet size in Venturi scrubbers. When Dinj is small or Dh is large, jet penetration is more likely to be not excessive, diminishing the deposition of the big droplets on the opposite wall. Thus, the actual collector droplet mean diameter would be bigger. On the other hand, when Dinj /Dh , is large, preferential droplet deposition may remove the big droplets from the core, with the overall effect of diminishing droplet Sauter mean diameter. The parameter Dinj /Dh is only geometrical, it must be investigated whether a more direct parameter such as film fraction could account better for the influence of preferential droplet deposition on the remaining droplet size. In a Pease-Anthony Venturi scrubber, the film is formed after the jet atomization, and deposition is much more important than entrainment during the first stages of film formation. The preferential deposition of a range of droplets would have a decisive influence on the final average size of the droplets that act as contaminant collectors. Thus, in a Venturi scrubber, the parameters that control jet atomization and droplet formation are not the only ones affecting actual droplet size. Fig. 7(a) and (b) shows film fraction (Ff ) as a function of jet penetration (**/Dh ) under different experimental conditions. The deposition of liquid on the throat of the equipment proved to be influenced significantly by the penetration of the jets. It can be observed that for the higher jet penetration fractions the highest film fractions were measured. Under these conditions, the droplets measured in the throat of the scrubber were smaller. This suggests that, jets with excessive penetrations, in addition to favoring improvement in the atomization process, by generating a greater number of smaller droplets, can be causing the preferential deposit of larger droplets on the walls opposite to the injection of
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
1 orifice; Vg = 59 m/s 2 orifices mode 1; Vg = 59 m/s 2 orifices mode 2; Vg = 59 m/s 3 orifices; Vg = 59 m/s 4 orfices; Vg = 59 m/s 1 orifices; Vg = 69 m/s 2 orifices mode 1; Vg = 69 m/s 2 orifices mode 2; Vg = 69 m/s 3 orifices; Vg = 69 m/s 4 orifices; Vg = 69 m/s
0.20 0.18 0.16
D32/do (-)
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1 orifice; Vg = 64 m/s 2 orifices mode 1; Vg = 64 m/s 2 orifices mode 2; Vg = 64 m/s 3 orifices; Vg = 64 m/s 4 orfices; Vg = 64 m/s 1 orifice; Vg = 74 m/s 2 orifices mode 1; Vg = 74 m/s 2 orifices mode 2; Vg = 74 m/s 3 orifices; Vg = 74 m/s 4 orifices; Vg = 74 m/s
0.16
D32/do (-)
0.14 0.12 0.10 0.08 0.06 0.04 0.02
0.00 0.2
0.3
0.4
0.5
0.6
0.7
0.8
**/Dh (-) Fig. 8. (a) Sauter diameter in function of jet the penetration fraction (**/Dh ), for gas velocity of 59 m/s and 64 m/s. (b) Sauter diameter in function of jet the penetration fraction (**/Dh ) for gas velocities of 64 m/s and 74 m/s.
the liquid. This deposition would be caused by the greater inertia of these droplets and favored by its proximity to the walls. The opposite occurs with less penetrating jets. The larger droplets tend to maintain their initial trajectory, due to inertia, while the small droplets are more easily carried by the gas turbulence, easily changing their initial trajectories. In the case of low jet penetration fractions, the larger droplets could flow freely on the air current without hitting the opposite wall since according to the atomization process described by Wu et al. [10], the larger droplets would form on the upper part of the jet. Through Figs. 7(a) and (b) and 8(a) and (b) one can also verify that the situations of larger deposits of liquid film, when the liquid was injected through 1 orifice and 2 orifices – mode 1, the size of the droplets flowing through the throat of the scrubber were smaller, suggesting the existence of preferential deposition. In an attempt to better quantify the influence that the deposition of droplets has on the remaining droplet size, the influence of the fraction of film measured experimentally was included in Eq. (8). In this way, the value Fe , which represents the quantity of liquid that is flowing in the form of droplets in the scrubber throat, was included in Eq. (8). Thus, the equation for estimating the size of the droplet considering the influence of the deposition of droplets on the scrubber walls would take the following form: D32 = 1.05 · Rel−0.21 · Weg−1.32 · Reg0.46 · do in which Fe = (1 − Ff ).
1 orifice 2 orifices mode 1 2 orifices mode 2 3 orifices 4 orifices
50
0
0.18
0.1
100
0
0.20
0.0
-15 %
150
0.8
**/Dh (-)
b
+15 %
200
D32experimental ( m)
a
1141
Dinj Dh
−0.51 · Fe2.63
(9)
50
100
150
200
D32estimated ( m) Fig. 9. Performance of the correlation proposed by Eq. (9).
Fig. 9 shows the comparison between the predicted size given by Eq. (9), to which was added the influence of the liquid film and the experimental results. Fig. 9 shows that the data were better adjusted by Eq. (9), and the droplet size estimated by the equation stayed within the range of ±15%. Using Eq. (9), it can be seen that the constants referring to the influence of the liquid injection configuration were equal to those adjusted in Eq. (8). This suggests that with an added term to include the film deposition effects, behavior with respect to position of the orifice on the throat was the same. The influence of the jet velocity on the reduction of droplet size was diminished while the effect of gas increased. This behavior can be seen in the value of the exponent referring to jet velocity, which went from −0.26 in Eq. (8) to −0.21 in Eq. (9). The influence of gas velocity increased from −1.64 to −2.18. These variations are a result of including the effect of deposition of liquid film on the walls of the equipment. Through the values of the adjusted parameters in Eq. (9), it can be observed that the film fraction deposited exercises a significant influence on droplet size as measured inside the scrubber. The value Fe raised to the highest power (2.63) and the improvement in the adjustment to the experimental data when comparing Fig. 6 with Fig. 9, means that preferential deposition of droplets of a certain size is of significant importance and must be considered in the prediction of the mean diameter of the droplets remaining in the core. These droplets, which flow along with the air current, are responsible for collecting the contaminants present in the gaseous current. 4. Conclusion Through the experimental results and the adjusted correlation for the estimate of droplet size, it can be concluded that: • Under the conditions in which the largest film fractions occur, the smallest size droplets also occur, suggesting the preferential deposition of larger-size droplets on the equipment walls. • The increase in jet velocity, as well as gas velocity contributed to the reduction of droplet size. • The configuration of the liquid injection proved to have a significant influence on the size of droplets. • The correlation proposed (Eq. (9)), in which the effect of the fraction of film that deposits on the wall was added, adjusted the experimental results satisfactorily.
1142
V.G. Guerra et al. / Chemical Engineering and Processing 50 (2011) 1137–1142
• Factors that are not usually taken into consideration in available correlations for the prediction of droplet size in Venturi scrubbers, such as the film fraction and the geometrical details of the liquid injection system, proved to be highly relevant to the final droplet size. Acknowledgements The authors are grateful to CNPq and FAPESP for financial support given to this work. Appendix A. Nomenclature
do D32 Dh Dinj Fe Ff Qg ** Ql No Vd Vg Vj Rel Reg Weg l l g
Diameter of orifice (m) Sauter mean diameter (m) Hydraulic diameter (m) Representative liquid injection throat diameter for a rectangular Venturi scrubber (m) Fraction of liquid flowing in the core of the Venturi throat (dimensionless) Film fraction (dimensionless) Volumetric gas flow rate (m3 /s) Liquid jet central penetration (m) Volumetric liquid flow rate (m3 /s) Number of orifices for liquid injection (dimensionless) Droplet velocity at atomization point (m/s) Gas velocity (m/s) Jet velocity (m/s) Reynolds number of liquid Reynolds number of gas Weber number Liquid viscosity (kg/m s) Liquid density (kg/m3 ) Gas density (kg/m3 ) Surface tension (N/m)
References [1] V.G. Guerra, J.A.S. Gonc¸alves, J.R. Coury, Experimental investigation on the effect of liquid injection by multiple orifices in the formation of droplets in a Venturi scrubber, J. Hazard. Mater. 161 (2009) 351–359. [2] J.A.S. Gonc¸alves, M.A.M. Costa, P.R. Henrique, J.R. Coury, Atomization of liquids in a Pease-Anthony Venturi scrubber. Part I. Jet dynamics, J. Hazard. Mater. 97 (2003) 267–279.
[3] J.A.S. Gonc¸alves, M.A.M. Costa, M.L. Aguiar, J.R. Coury, Atomization of liquids in a Pease-Anthony Venturi scrubber. Part II. Droplet dispersion, J. Hazard. Mater. 116 (2004) 147–157. [4] J.A.S. Gonc¸alves, D. Fernández Alonso, M.A.M. Costa, B.J. Azzopardi, J.R. Coury, Evaluation of the models available for the prediction of pressure drop in Venturi scrubbers, J. Hazard. Mater. 81 (2001) 123–140. [5] D.F. Alonso, J.A. Gonc¸alves, B.J. Azzopardi, J.R. Coury, Drop size measurements in Venturi scrubbers, Chem. Eng. Sci. 56 (2001) 4901–4911. [6] M.A.M. Costa, P.R. Henrique, J.A.S. Gonc¸alves, J.R. Coury, Droplet size in a rectangular Venturi scrubber, Braz. J. Chem. Eng. 21 (2004) 335–343. [7] A.M. Silva, J.C.F. Teixeira, S.F.C.F. Teixeira, Experiments in large scale Venturi scrubber. Part II. Droplet size, Chem. Eng. Process. 48 (2009) 424–431. [8] A.M. Silva, J.C.F. Teixeira, S.F.C.F. Teixeira, Experiments in large scale Venturi scrubber. Part I. Pressure drop, Chem. Eng. Process. 48 (2009) 59–67. [9] S. Viswanathan, C.C.S.T. Pierre, A.W. Gnyp, Jet penetration measurements in a Venturi scrubber, Can. J. Chem. Eng. 61 (1983) 504–508. [10] P-K. Wu, K.A. Kirkendall, R. Fuller, A.S. Nejad, Breakup processes of liquid jets in subsonic crossflows, J. Propul. Power 13 (1997) 64–73. [11] S. Nukiyama, Y. Tanasawa, Experiment on atomization of liquid by means of air stream, Trans. Soc. Mech. Eng. Jpn. 4 (1938) 86–93. [12] R.H. Boll, L.R. Flais, P.W. Maurer, W.L. Thompson, Mean drop size in a full scale Venturi scrubber via transmissometer, J. Air Pollut. Control Assoc. 24 (1974) 934–938. [13] A. Tratining, G. Brenn, T. Strixner, P. Fankhauser, N. Laubacher, M. Stranzinger, Characterization of spray formation from emulsions by pressure-swirl atomizers for spray drying, J. Food Eng. 95 (2009) 126–134. [14] A.H. Lefebvre, Atomization and Sprays, Hemisphere Publishing Corporation, New York, 1989. [15] E.J. Lavernia, Y. Wu, Spray Atomization and Deposition, John Wiley & Sons, Chichester, 1996. [16] H.F. Liu, W.F. Li, X. Gong, X.K. Cao, J.L. Xu, X.L. Chen, Y.F. Wang, G.S. Yu, F.C. Wang, Z.H. Yu, Effect of liquid jet diameter on performance of coaxial two-fluid airblast atomizers, Chem. Eng. Process. 45 (2006) 240–245. [17] W. Ambrosini, P. Andreussi, B.J. Azzopardi, A physically based correlation for drop size in annular flow, Int. J. Multiphase Flow 17 (1991) 497–507. [18] B.J. Azzopardi, Drops in annular two-phase flow, Int. J. Multiphase Flow 23 (1997) S1–S53. [19] M.J.H. Simmons, T.J. Hanratty, Droplet size measurements in horizontal annular gas–liquid flows, Int. J. Multiphase Flow 27 (2001) 861–883. [20] A. Al-Sarki, T.J. Hanratty, Effect of pipe diameter on the drop size in a horizontal annular gás–liquid flow, Int. J. Multiphase Flow 28 (2002) 1617–1629. [21] V. Cleary, P. Bowen, H. Witlox, Flashing liquid jets and two-phase droplet dispersion. I. Experiments for derivation of droplet atomization correlations, J. Hazard. Mater. 142 (2007) 786–796. [22] K. Krishna, T.K. Kim, K.D. Kihm, W.J. Rogers, M.S. Mannan, Predictive correlations for leaking heat transfer fluid aerosols in air, J. Loss Prevent. Proc. 16 (2003) 1–8. [23] K.D. Kihm, G.M. Lyn, S.Y. Son, Atomization of cross-injecting sprays into convective air stream, Atomization Spray 5 (1995) 417–433. [24] B.K. Park, J.S. Lee, K.D. Kihm, Comparative study of twin-fluid atomization using sonic or supersonic gas jets, Atomization Spray 6 (1996) 285–304. [25] K.D. Kihm, N. Chigier, Effect of shock waves on liquid atomization of a twodimensional airblast atomizer, Atomization Spray 1 (1995) 113–136. [26] K.J. Hay, Z-C. Liu, T.J. Hanratty, Relation of deposition to drop size when the rate is nonlinear, Int. J. Multiphase Flow 22 (1996) 829–848. [27] L. Bayvel, Z. Orzechnowsy, Liquid Atomization, Taylor & Francis, Washington, D.C., 1993. [28] G.G. Nasr, A.J. Yule, L. Bendig, Industrial Sprays and Atomization, SpringerVerlag, London, 2002.