Environ. Sci. Technol. 1999, 33, 4492-4494
Electrostatic Precipitator as a Generator Rather Than a Remover of Small Droplets J. C. M. MARIJNISSEN,* W. OOSTRA, A. M. MOLLINGER, AND P. H. W. VERCOULEN Faculty of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
The electrostatic precipitator (ESP) is used all over the world to clean particles from contaminated industrial and domestical air. Recent observations and experiments however are disturbing. These indicate that, when capturing an oil mist or possibly droplets in general, phenomena occur that can cause the ESP to generate rather than remove (sub)micron particles. At spots where the electric field strength is high enough, collected droplets deform into a conical shape, the so-called Taylor cone. At the tip of this cone a jet is formed that breaks up in an enormous number of (sub)micron-sized droplets. In the ESP, this occurs at the charging corona wires and at sharp points on the collector plates. Because a part of the ESP energy is thus being used to spray droplets electrically, less energy can be utilized for ionization to charge incoming particles. Simultaneously, the active field strength between the collector plates decreases. These factors result in a substantial decrease in collection efficiency over the total size range. In addition to this overall decrease in efficiency, particles in the order of 1 µm are generated from the tip of the developed Taylor cones. Most of these generated droplets will be trapped in the ESP, but droplets formed close to the exit of the plates, depending on the configuration of the ESP, will be discharged from the precipitator. Thus, for particles of this size, the ESP is a particle generator rather than a particle remover.
Introduction During an efficiency test of a commercial parallel-plate twostage ESP with an air flow rate of 2000 m3/h, a remarkable detail was observed. Using an oil mist with a mass median diameter of 2 µm and a geometric standard deviation of about 2 as the test aerosol, the collection efficiency on a number base as a function of the droplet size was measured with an optical particle counter (Royco 245, calibrated with monodisperse latex particles). Although the overall mass collection was reasonable, the test showed that the removal efficiency for the droplets of approximately 0.6 µm was very low. By using the ESP without a grounded gauze prefilter and without a fine carbon after-fllter, the collection efficiency for oil droplets of approximately 0.6 µm was negative. In other words, the ESP generated droplets of this size (Figure 1). It was also noticed that, with the grounded prefilter in place, this filter became wet with oil (droplets) on the inside. Different researchers describe the formation of condensation nuclei resulting from gas-phase reactions between * Corresponding author phone: + 31 (0)15 278 4368; fax: + 31 (0)15 278 4452; e-mail:
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FIGURE 1. Collection efficiency on a number base of an oil mist in an ESP.
FIGURE 2. Schematic diagram of the ESP; wire diameter 100 µm. products from corona discharges (1-4). Since these nuclei are in the nanometer range (2), it seems unrealistic to attribute the measured low collection efficiency to the phenomena discribed above. Cucu and Lippold (5) mention that the efficiency of an ESP decreases with time for oil mist collection. They impute this decrease to the formation of an oil film on the originally dry plates of the ESP. This oil film generates very fine particles due to electrohydrodynamic forces. Unfortunately, the collection efficiency as a function of droplet diameter is not presented.
Experimental Observations and Explanation To get a first impression on the droplet-generating mechanism, some preliminary tests were performed. For this purpose, a commercially available air cleaning device was used. The air cleaner consisted of a coarse filter, a corona wire, an electrostatic precipitator, a filter, and an air blower. The coarse filter was removed to get a clear view of the ESP. Figure 2 gives a schematic diagram of the precipitator. The wires and the charged plates have a potential difference of 5.3 kV with respect to the grounded plates. With a nebulizer, oil is sprayed into the ESP. Using a macroscope focused on one of the wires, the processes occurring there were followed. Observation shows that small droplets collect on the corona wire where they are converted into a conical shape when the voltage is applied (Figure 3b). These conically shaped droplets move slightly along the wire. As soon as the voltage is taken off, the conical shape disappears and a normal droplet remains (Figure 3a). The shape of the conical drop has a remarkable resemblance to the so-called Taylor cone. When an electrostatic field of sufficient strength is applied to a liquid droplet, the droplet deforms into a conical shape; this is commonly called a Taylor cone (Figure 3c). In 1600, this phenomenon was already observed by William Gilbert, 10.1021/es990093o CCC: $18.00
1999 American Chemical Society Published on Web 11/06/1999
FIGURE 3. (a) Droplet on wire without field. (b) Droplet on wire with field (wire diameter is 100 µm). (c) A spraying Taylor cone. (d) A Taylor cone on the plate of an ESP (plate thickness 1 mm). who described how a droplet of water was deformed into a conical shape when a charged amber rod was brought into its proximity (6). During the twentieth century, several researchers reported on the Taylor cone. They described the formation of very fine aerosol particles from the jet at the tip of the cone. Taylor studied the conditions under which a conically shaped liquid surface can exist by considering the influence of the forces of the electric field and the surface tension of the liquid (7). He derived a mathematical description for the limiting case of no ejection of droplets. Reviews of liquid atomization by electrical means, including the different spraying modes, are given by Cloupeau and PrunetFoch (8) and by Grace and Marijnissen (9). The emission of small droplets from the jet at the cone in the so-called conejet mode can be surprisingly high, i.e., on the order of hundreds of thousands per second. The size of the droplets can vary from nanometers to many micrometers, depending mainly on the liquid properties and, in the steady-state case, the flow rate. The droplets carry a high charge, and the mean droplet size decreases with increasing liquid conductivity. Complete agreement between different researchers on the jet breakup mechanism has yet to be reached. That different spray regimes exist seems likely. In 1965, Akazaki reports sharp water points that form on high voltage power lines and demonstrates the similarity of the shapes of individual corona pulses from water drop points hanging from a conductor to those from metal points (10). Although not mentioned as such, he clearly refers to Taylor cones. It is therefore clear that high voltage charging wires in an ESP can behave as aerosol generators when liquid droplets are collected. The emitted droplets carry a high charge, some 10 000 elementary charges for an oil droplet of 1 µm, which may be close to the so-called Rayleigh charge limit (11-13). This charge certainly is sufficient to ensure that the droplets entering the collection section in the ESP are trapped. This high charge also enables droplets that happen to be on the upstream side of the air flow close to the charging wire (Figure 2), where the electric field strength is expected to be around
FIGURE 4. (Curve A) Normal filter efficiency of an ESP. (Curve B) Filter efficiency of an ESP with oil on the wires. 106 V/m, to move in the direction of the grounded prefilter. This movement explains why the filter becomes wet on the inside. Since energy is consumed when spraying droplets from the wire and subsequently capturing the emitted droplets on the collection plates, it is to be expected that the overall collection efficiency of an ESP decreases when cleaning an oil mist. To examine this hypothesis, the ESP of Figure 2 was again used. In the first test, the collection efficiency as a function of the particle size for normal laboratory air was measured. The size distribution and concentration were measured before and after the apparatus with an optical particle counter. From the simple equation η ) 1 - p, in which η is the collection efficiency and p is the penetration, Figure 4, curve A was constructed. Subsequently, small amounts of paraffin oil were applied to the charging corona wires with a small brush. The collection efficiency as a function of the particle diameter was again measured. The results are shown in Figure 4, curve B. However, to a certain extent, this decrease VOL. 33, NO. 24, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in efficiency could also be due to partly covering the corona wires with small amounts of paraffin oil, reducing there the electric field strength. It certainly does not explain the selective low efficiency indicated in Figure 1. To investigate this, the rear side of the ESP was removed, and the back edge of one of the high voltage plates was roughened with a file to create sharp irregularities observable by a macroscope. Paraffin oil was brushed onto the roughened plate. With the high voltage switched on, Taylor cones became visible on the irregularities (Figure 3d). It is clear therefore that Tayler cones develop at sharp edges of the collection plates. Droplets generated close to the exit of the ESP will no longer be captured and thus exit from the ESP. Thus, the ESP has become a droplet generator rather than a droplet remover. For the liquid studied here, the diameter of the droplets produced by the ESP are in the respirable size range. To verify if these cones really spray droplets comparable with the ones measured during the efficiency test of the ESP as described in the Introduction, corn oil and mixtures of corn oil with ethanol and butanol (with known good spraying characteristics) were brushed onto the roughened side of the high voltage plate at the end of the ESP. In all cases, the number of particles (droplets) measured with the high voltage switched on was up to several decades higher than that measured with the high voltage off. So droplets are generated. The aerodynamic size varied between some tenths of a micrometer to a few micrometers (measured with Aerosizer, Amherst Process Instruments, Inc.; assumed liquid density of 1000 kg/m3). The mode was between 0.6 and 1.2 µm, which corresponds to Figure 1. The results indicate that spraying does not take place in a stable mode, which is logical since there is no steady flow situation. If a mixture is brushed onto a smooth part of the high voltage plate, the phenomenon does not occur, which confirms that sharp edges are necessary to create a high field. For the liquid studied here, the diameter of the droplets created is in the respirable range. In addition, these small particles can remain dispersed in the atmosphere for a long time and thus be transported over very long distances.
Significance and Implications The phenomenon of electrostatic spraying occurring in ESPs has serious health and environmental consequences. It should be noted that particles of about 1 µm penetrate into the deepest part of the lungs. Furthermore, because of their small size, they can be transported over long distances in the air. Many tens of thousands of ESPs are used all over the world both in industry and domestic situations, with many specifically used to clean liquid droplets from the air. Industrial examples are the cleaning of acid mists from sulfuric acid and phosphere acid plants and the elimination
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from oil mist in machine shops. Actually the test as described in the Introduction was on the ESP of a machine shop (1416). Masuda and Hosokawa mention the use of a wet ESP with water spraying or a water-irrigated collecting electrode, e.g., for a utilities’ coal-fired boiler. ESP is also in wide use for air cleaning in indoor environments such as offices, homes, hospitals, etc. The described phenomenon could be typically encountered in places where spray cans are commonly used, such as by hairdressers and beauticians. In view of these findings, it would seem prudent to further study the described phenomenon and behavior for both design purposes and use of electrostatic precipitators.
Acknowledgments The authors acknowledge the following people for their contributions to this work: Sheldon K. Friedlander, S. M. Lemkowitz, R. A. Roos, D. J. Brunner, W. R. Passwaters, and D. Bemer, who provided us the data and results of the test as mentioned in the Introduction.
Literature Cited (1) Lapeyre, R.-M.; Peyrous, R. Environ. Technol. Lett. 1981, 2, 2938. (2) Nolan P. J.; Kuffel, E. Geofis. Pura Appl. 1957, 36, 201-210. (3) Keskinen, J.; Janka, K.; Lehtimaki, M.; Graeffe, G.; Kulmala, V. J. Aerosol Sci. 1986, 17 (3), 647-649. (4) Allen, N. L.; Coxen, P.; Peyrous, R.; Teisseyre, Y. J. Phys. D: Appl. Phys. 1981, 14, L207-L209. (5) Cucu, D.; Lippold, H. J. J. Electrost. 1985 17, 109-112. (6) Gilbert, W. De Magnete; Mottelay, P. F., Translator; Dover Publications: New York, 1958; p 89. (7) Taylor, G. I. Proc. R. Soc. London 1964, A280, 383. (8) Cloupeau, M.; Prunet-Foch, B. J. Aersol Sci. 1994, 25 (6), 10211036. (9) Grace, J. M.; Marijnissen, J. C. M. J. Aerosol Sci. 1994, 25 (6), 1005-1019. (10) Akazaki, M. IEEE Trans. 1965, No. PAS-84, 1-8. (11) Fernandez de la Mora, J.; Loscertales, I. G. J. Fluid Mech. 1994, 260, 155-184. (12) Gomez, A.; Tang, K. Phys. Fluids 1994, 6 (1), 404-414. (13) Ganan-Calvo, A. M.; Davila, J.; Barrero, A. J. Aerosol Sci. 1997, 28 (2), 249-275. (14) Rose, H. E.; Wood, A. J. An Introduction to electrostatic precipitation in theory and practice; Constable & Company Ltd: London, 1966; p 24. (15) Katz, J. The art of electrostatic precipitation; Precipitator Technology, Inc: Munhall, PA, 1979; pp 5-6. (16) Masuda, S.; Hosokawa, S. In Handbook of electrostatic processes; Chang, J.-S., Kelly, A. J., Crowley, J. M., Eds.; Marcel Dekker: New York, 1995; pp 441-479.
Received for review January 27, 1999. Revised manuscript received June 15, 1999. Accepted September 21, 1999. ES990093O
