A STUDY OF A WIRE-PLATE ELECTROSTATIC PRECIPITATOR OPERATING IN THE REMOVAL OF POLYDISPERSED PARTICLES

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1 Brazilian Journal of Chemical Engineering ISSN Printed in Brazil Vol. 21, No , Aril - June 24 A STUDY OF A WIRE-PLATE ELECTROSTATIC PRECIPITATOR OPERATING IN THE REMOVAL OF POLYDISPERSED PARTICLES S. W. Nóbrega 1, M. C. R. Falaguasta 2 and J. R. Coury 2* 1 Deartamento de Engenharia Química, Universidade Federal de Alagoas, Phone +(55) (82) , Fax +(55) (82) , Camus A. C. Simões, BR 14, km 14, 572-9, Maceió - AL, Brazil. swn@ctec.ufal.br 2 Deartamento de Engenharia Química, Universidade Federal de São Carlos, Phone +(55) (16) , Fax +(55) (16) , Rodovia Washington Luís, km 235, Cx. P. 676, CEP 135-9, São Carlos - SP, Brazil. jcoury@ower.ufscar.br (Received: February 13, 23 ; Acceted: October 7, 23) Abstract - Performance of a wire-late electrostatic reciitator in the removal of articles with a wide article size distribution was evaluated. The exerimental rig utilized consisted of a set of three reciitators whose dimensions could be varied. It was observed that reciitator erformance increased with an increase in the ga between the collecting lates, which resulted in a substantial increase in energy consumtion er volume of cleaned gas. The grade efficiency redicted by correlations from the literature did not match the exerimental measurements, and a new correlation is roosed. Keywords: electrostatic reciitator, gas cleaning, article collection. INTRODUCTION The electrostatic reciitator is widely emloyed in the control of articles emissions in different industrial rocesses. In this equiment, the articles in the gaseous current are electrically charged and searated from the gas under the influence of an electric field. Project and oeration arameters have a big influence on the erformance of a reciitator. Although a great deal of research has been done on the influence of these arameters, contradictory conclusions are often reorted. The shae of the reciitator has considerable influence on its erformance. Many researchers (e.g. Leonard et al., 19; Petersen, 1981; Riehle and Löffler, 1992a) discuss this influence, but the conclusions are contradictory. Chang and Bai (2) used a mathematical model to show that the wider reciitators do not function as well. Navarrete et al. (1997), however, verified that reciitators with wider ducts were more efficient in the removal of high resistivity materials. The erformance of electrostatic reciitators is, in general, evaluated under different electric oerational conditions, which is erhas what leads to the different conclusions observed in the literature. The comlexity of the henomenon is oorly addressed in the models available for redicting collection efficiency, articularly in the submicron article size range. The objective of this work was to study the erformance of the electrostatic reciitator in the removal of a articulate with a wide range of sizes, under different oerational conditions and to comare the results for collection efficiency with redictions by available theoretical models. *To whom corresondence should be addressed

2 276 S. W. Nóbrega, M. C. R. Falaguasta and J. R. Coury Collection Efficiency In the literature there are several correlations for rediction of collection efficiency in electrostatic reciitators. However, their use has been limited by emirical constants that are difficult to estimate as well by their comlexity in some cases (Zhao and Pfeffer, 1996). The classic model for collection efficiency, known as the Deutsch equation, Equation 1, is still frequently used to redict the erformance of reciitators due to its simlicity (Riehle, 1997). In this model it is assumed that the rofile for concentration of articles in any traverse section of the duct is uniform. wtha wthlne Q vs o di 1e 1e (1) In this equation, w th is the theoretical migration velocity, which is obtained from the use of balance equations on a article, assuming Stokes law, and is given by w th Q ECu (2) 3 d To calculate migration velocity, Riehle (1997) suggests using the Cochet equation (Equation 3) for estimation of the saturation charge of the articles, Q. The electric field, still according to Riehle, can be estimated by the ratio of alied otential to the distance of the wire from the late (s). Cb o v C(x, y) ex(x') 2 D x with wx vy v X' 4D x 2 (5) (6) where y and x are the axial and traverse distances in resect to the gas flow, resectively. From the definition of collection efficiency, it follows that di 2 Pe( De).5 Pe 1 4De d 4De (7) 1 e where the Pèclet, Pe, and Deutsche, De, numbers are given by w.s Pe (8) D th w.l De (9) th NE v.s o Equation 7 is therefore the theoretical model of Zhibin and Guoquan (1994) for collection efficiency in electrostatic reciitators r 1 2 o d 2 r 2 Q 1 d E 1 d (3) MATERIALS AND METHODS Exerimental Setu Attemting to avoid the assumtion of a constant concentration rofile in the Deutsch model, several other researchers suggested new models that consider the diffusivity of the article in the flowing gas. Zhibin and Guoquan (1994) included the effect of diffusion in their develoment of an analytic model for collection efficiency in which the equation of bidimensional transort of the articles as a function of concentration (Equation 4) was analytically solved. Equation 5 gives the solution resented by the authors. 2 C C 2 o th D v w y x C y (4) The exerimental system used in this study consisted of three electrostatic reciitators of the wire-late tye, whose main dimensions are given in Table 1. A schematic view of one of the three modules is shown in Figure 1. Each reciitator consisted of a rectangular box built of acrylic and PVC, with the lateral walls covered by grounded coer lates. These lates constituted the collection electrodes. The discharge electrodes were stainless steel wires, stretched halfway and orthogonal between the collecting lates. The distance between the electrodes (2c) could be varied for each reciitator, according to the values resented in Table 1. Brazilian Journal of Chemical Engineering

3 A Study of a Wire-Plate Electrostatic Preciitator 277 Table 1: Main dimensions of the reciitators utilized. Preciitator s (cm) c (cm) 3.3; 5.; 1. 5.; 7.5; ; 1.; 2. 2r SE (mm) L NE (cm) h NE (cm) high-voltage suly blower owder feed samling ort (a) 2s samling ort 2c h NE 2r SE L NE (b) Figure 1: (a) Scheme of a module of the exerimental unit; (b) Shae of the reciitator Accum. fraction d (m) Figure 2: Size distribution of the articulate matter utilized. Brazilian Journal of Chemical Engineering Vol. 21, No. 2, , Aril - June 24

4 278 S. W. Nóbrega, M. C. R. Falaguasta and J. R. Coury Particulate The articulate material used in the exerimental tests was a hoshatic concentrate with an electric 8 resistivity of 8.31 Wm, a dielectric constant of and a density of 3.1 kg/m 3. The size analysis of this owder was obtained utilizing an equiment, the Malvern Mastersizer, which determines the article size distribution by laser diffraction. The size distribution of the owder utilized in this work is shown in Figure 2. Exerimental Tests The methodology used in the tests consisted in initially setting u the exerimental system with the levels of the variables to be tested: the blower was started and the air flow rate adjusted so that the gas velocity inside the reciitator reached the required value. The test was then started with the feeding of the articulate matter. After reaching the steady state (aroximately five minutes), the owder concentration was measured at the entrance of the system by isokinetic samling. Soon after, a new samling was taken as the matter left the reciitator to obtain the concentration of articles at the exit of the system. The grade efficiency of collection ( di ) for each test was obtained from the size analysis of the articulate matter retained in the filters used in the samlings. The articulate matter samled from the reciitator was re-disersed in a solution (deionized water lus disersant agent) utilizing an ultrasonic bath. The susension was then ut in a Malvern Mastersizer, where size was analyzed. From the results obtained in these analyses, together with the article concentrations at the entrance and exit of the system, grade efficiency was calculated with the use of the following equation S csd i di 1 E ce d i RESULTS AND DISCUSSION (1) Influence of Duct Width on Preciitator Performance Figure 3 shows the exerimental results of the erformance of two reciitators with different widths 2s, oerating with the same secific collection area (A/Q), for three values of alied electric field. All the curves show an identical behavior: efficiency reaches a minimum in the vicinity of the 1 m size range. Efficiency in the removal of larger articles is well understood and the electric force created by the reciitator is reliably correlated with removal forces. Some comrehensive reviews on the subject can be found in the literature (Oglesby and Nichols, 1978; Parker, 1997). Conversely, efficiency in the submicron range does not have a clear theoretical descrition and is still oorly understood. In this size range, the effects of turbulent diffusion and of discontinuity of the flowing medium can revail. Figure 3 also shows that an increase in the width of the duct increased articles removal, esecially for the electric field of low intensity. The fact that wider reciitators function better is known as the "non- Deutschian" henomenon (Chang and Bai, 2). Riehle and Löffler (1992b) attribute this behavior to the fact that the electric field increases in roximity to the collection lates with the increase in the width of the duct, intensifying the collection of articles. Although the results show that wider reciitators function better, it is imortant to observe that the costs of installation and oeration for these reciitators are higher than those for narrower ducts. This is because the increased width of the duct requires a roortional increase in the electric otential to maintain the same electric field. Consequently, it is necessary to utilize a high-voltage source of larger caacity, increasing the initial cost of installation. Regarding the cost of oeration, the ower consumed by unit of flow rate of treated gas increases with the width of the duct, as can be observed in Figure 4, where the values in ercentage reresent the increase in consumtion of electricity. Therefore, an increase in the width of the duct might not be economically viable, esecially when the secific collection area (A/Q) is small. Another form of evaluating the influence of distance between the collection lates (2s) is by comaring reciitator erformance for different duct widths with the same consumtion of electricity ower er unit of treated gas flow rate. The results obtained in these tests are resented in Figure 5. Observing the behavior of these curves, it can be observed that for the same amount of ower consumed, erformance of the reciitators imroves as the distance between the lates decreases. This is due to the fact that the increase in duct width requires more energy to increase the migration velocity of the articles in order to comensate for the increase in the distance to be traveled. Brazilian Journal of Chemical Engineering

5 A Study of a Wire-Plate Electrostatic Preciitator 279 di d (m) E = 3 kv/cm 2s =.1 m 2s =.2 m di d (m) E= 4 kv/cm 2s =.1 m 2s =.2 m di d (m) E = 5 kv/cm 2s =.1 m 2s =.2 m Figure 3: Influence of 2s on collection efficiency: A/Q = 12 s/m; s/2c =.5 Brazilian Journal of Chemical Engineering Vol. 21, No. 2, , Aril - June 24

6 2 S. W. Nóbrega, M. C. R. Falaguasta and J. R. Coury s =.1 m 2s =.2 m 22% 12 P elet (W/m 3 /s) % 4 199% E (kv/cm) Figure 4: Power consumtion er unit of gas flow rate: A/Q = 12 s/m; S/2c =.5. di P = Watts/m 3 /s 2s =.1 m 2s =.15 m 2s =.2 m d (m) di d (m) P = 3 Watts/m 3 /s 2s =.15 m 2s =.2 m Figure 5: Influence of the distance between the lates on efficiency for reciitators with the same ower consumtion er unit of gas flow rate. Brazilian Journal of Chemical Engineering

7 A Study of a Wire-Plate Electrostatic Preciitator 281 Therefore, analyzing the influence of duct width (2s) on the erformance of the reciitator, it becomes evident that an increase in this arameter generally causes a higher energy consumtion without, however, resulting in a significant imrovement in reciitator erformance. Comarison Between Theoretical Models and Exerimental Data on Grade Collection Efficiency Figure 6 comares values for grade collection efficiency, obtained exerimentally with redictions by the models of Deutsch and Zhibin and Guoquan, Equations 1 and 7, resectively. As can be seen in these figures, those correlations did not reresent well the exerimental data. An alternative roosal to the solution given by Zhibin and Guoquan is shown in Equation 11, which is also a solution to Equation 4. In this new equation, it is assumed that the concentration of articles is smaller at the center of the reciitator duct and increases moving towards the collection lates, which seems quite reasonably in agreement with the exerimental data obtained by Schmid and Umhauer (1998) and shown in Figure 7. Riehle and Löffler (1993) also deicted this behavior in a number of hotograhs taken from inside a wire-late reciitator. 1.5 v C(x, y) wx ycob ex(x') D x (11) where X' is given by Equation 6. The equation for collection efficiency is given in this case by Pe 1 Pe De 2 4De di 1 e d 1.5 De (12) di (a) d (m) Exerimental Deutsch (Eq. 1) Zhibin-Guoquan (Eq. 7) di (b) d (m) Exerimental Deutsch (Eq. 1) Zhibin-Guoquan (Eq. 7) Figure 6: Comarison between theoretical models and exerimental data for collection efficiency 2s =.2; s/2c = 1 (a) v o = 1.5 m/s and E = 5 kv/cm; (b) v o = 1. m/s and E = 3 kv/cm. Brazilian Journal of Chemical Engineering Vol. 21, No. 2, , Aril - June 24

8 282 S. W. Nóbrega, M. C. R. Falaguasta and J. R. Coury after the 3 rd electrode after the 4 th electrode after the 5 th electrode after the 6 th electrode J N /J N, y (mm) Figure 7: Profile of article flow in consecutive crossections of the reciitator for V = -45 kv, v o = 1. m/s, d = 1. m, 2s =.2 m, from Schmid and Umhauer (1998) The value to be attributed to the diffusion coefficient, D, was a matter of concern. No reliable theoretical correlation is currently available and the one used by Zhibin and Guoquan (1994), in which D was indeendent of article size, did not seem reasonable. For simlicity, D was assumed to be inversely roortional to article size, and the roortionality constant was obtain from the exeriment. The resulting exression is shown in Equation 13. The values obtained for the range of article diameters between.1 and 1 m varied between.1 and 3 m 2 /s, which according to Cooerman aud Licht (1988), is in agreement with the tendencies observed. D 7 2x1 (m 2 /s) (13) d for d in meters. A comarison between exerimental results and the new correlation (Equation 12) in resented in Figures 8, 9 and 1. It can be observed that the model reresents the exerimental behavior well, resonding satisfactorily to the changes in the oerational conditions and reciitator shae. 95 di s =.1 m: eq 12 exerim. 2s =.15 m: eq 12 exerim. 2s =.2 m: eq 12 exerim. d (m) Figure 8: Comarison between theoretical rediction (Equation 12) and exerimental data for grade collection efficiency. E = 4 kv/cm, v o = 1.5 m/s and s/2c = 1.. Brazilian Journal of Chemical Engineering

9 A Study of a Wire-Plate Electrostatic Preciitator di v o =.5 m/s: eq 12 exerim. v o = 1. m/s: eq 12 exerim. v o = 1.5 m/s: eq 12 exerim. d (m) Figure 9: Comarison between theoretical rediction (Equation 12) and exerimental data for grade collection efficiency. E = 5 kv/cm, 2s =.2 m and s/2c = 1.. di E = 3 kv/cm: eq 12 exerim. E = 4 kv/cm: eq 12 exerim. E = 5 kv/cm: eq 12 exerim. d (m) Figure 1: Comarison between theoretical rediction (Equation 12) and exerimental data for grade collection efficiency. v o =1.5 m/s, 2s =.2 m and s/2c = 1.. CONCLUSIONS The exerimental evidence resented here in has shown that an increase in duct width (2s) resulted in an increase in collection efficiency, but also in a larger consumtion of energy. The models of Deutsch and Zhibin and Guoquan (1994) did not redict reciitator efficiency in the submicron article size range. A new correlation (Equation 12), which reresented the exerimental behavior very well for different oerational conditions, is roosed. NOMENCLATURE A Total collecting area, m 2 2c Distance between two discharge electrodes, m c e Mass concentration of articles at the entrance, kg/m 3 c s Mass concentration of articles at the exit, kg/m 3 Cu Cunningham sli correction factor, dimensionless De Deutsch number, dimensionless Brazilian Journal of Chemical Engineering Vol. 21, No. 2, , Aril - June 24

10 284 S. W. Nóbrega, M. C. R. Falaguasta and J. R. Coury d Particle diameter, m D Particle diffusivity, m 2 /s E Electric field, V/m L NE Length of the collecting late, m Pe Pèclet number, dimensionless Q Gas volumetric flow rate, m 3 /s Q Particle saturation charge, C s Distance between the discharge and collecting electrodes, m 2s Duct width, m v o Gas velocity, m/s w th Migration velocity, m/s E d i Fraction of articles with diameter d i at the entrance of the system, % S d i Fraction of articles with diameter d i at the exit of the system, % o Permittivity of the vacuum, x 1 As/Vm r Dielectric constant of the articles, dimensionless di Grade efficiency, % Mean free ath of the ions, m Gas viscosity, Ns/m 2 ACKNOWLEDGEMENTS The authors would like to thank CAPES and PRONEX-FINEP for the financial suort that made this work ossible. REFERENCES Chang, C.L. and Bai, H., Effects of Some Geometric Parameters on the Electrostatic Preciitator Efficiency at Different Oeration Indexes, Aerosol Science and Technology, vol. 33, (2). Leonard, G. and Mitchner, M., Self, S.A, Particle Transort in Electrostatic Preciitators, Atmosheric Environment, vol. 14, (19). Licht, W., Air Pollution Control Engineering - Basic Calculations for Particulate Collection, 2 nd Ed., Marcel Dekker, New York (1988). Navarrete, B. and Cañadas, L., Cortés, V., Salvador, L., Galindo, J., Influence of Plate Sacing and Ash Resistivity on the Efficiency of Electrostatic Preciitators, Journal of Electrostatics, vol. 39, (1997). Oglesby, S. and Nichols, G.B., Electrostatic Preciitation, Pollution Engineering and Technology, Marcel Dekker, New York, NY (1978). Parker, K.R., Alied Electrostatic Preciitation, Blackie Academic & Professional, New York, NY (1997). Petersen, H.H., New Trends in Electrostatic Preciitation: Wide Duct Sacing, Precharging, Pulse Energization, IEEE Transactions on Industry Alications, vol. IA 17, no. 5, (1981). Riehle, C., Basic and Theoretical Oeration of ESPs. In: Parker, K.R., ed., Alied Electrostatic Preciitation, Cha.3, , Blackie Academic & Professional, New York, NY (1997). Riehle, C. and Löffler, F., Electrical Similarity Concerning Particle Transort in Electrostatic Preciitators, Journal of Electrostatics, vol. 29, (1992a). Riehle, C. and Löffler, F., The Effective Migration Rate in Electrostatic Preciitators, Aerosol Science and Technology, vol. 16, (1992b). Riehle, C. and Löffler, F., Reflections on Similarity Laws Concerning Particle Transort in Electrical Preciitators, Powder Technology, vol. 77, (1993). Schmid, H.J. and Umhauer, H., In-situ Measurement of Local Particle Fluxes in a Laboratory-scaled ESP. 7th. International Conference on Electrostatic Preciitation, 2-25 Setember, Kyongju, Korea (1998). Zhao, Z.M. and Pfeffer, R., A Semi-emirical Aroach to Predict the Total Collection Efficiency of Electrostatic Preciitators, Chemical Engineering Communications, vol , (1996). Zhibin, Z. and Guoquan, Z., Investigations of the Collection Efficiency of an Electrostatic Preciitation with Turbulent Effects, Aerosol Science and Technology, vol. 2, (1994). Brazilian Journal of Chemical Engineering