MEASUREMENT OF ELECTRIC CHARGE OF AEROSOL PARTICLES GENERATED BY VARIOUS METHODS

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MEASUREMENT OF ELECTRIC CHARGE OF AEROSOL PARTICLES GENERATED BY VARIOUS METHODS Yasuo KOUSAKA, Kikuo OKUYAMA, Motoaki ADACHIand Kenji EBIE Department of Chemical Engineering, University of Osaka

1 MEASUREMENT OF ELECTRIC CHARGE OF AEROSOL PARTICLES GENERATED BY VARIOUS METHODS Yasuo KOUSAKA, Kikuo OKUYAMA, Motoaki ADACHIand Kenji EBIE Department of Chemical Engineering, University of Osaka Prefecture, Sakai 591 The charge of aerosols generated by various methods was measured by a visual experimental technique. The results obtained are as follows: 1) Aerosol particles generated by condensation after evaporation almost reach the Boltzmanncharge distribution whenions are mixedas condensation nuclei, but have a lower charge when no ions are mixed. 2) The charge of aerosol particles generated by chemical reaction depends on the aerosol material and the kinds of reaction. Metal aerosol particles generated by combustion are highly charged, but particles of tobacco smoke are not highly charged. Aerosol particles of ammoniumchloride generated by chemical reaction, on the other hand, are not charged at all. 3) The charge of aerosols dried after atomization seems to be affected by the state of solution or suspension in nebulizer. Di-octyl sebacate particles are almost uncharged, sodium chloride particles are highly charged, and polystyrene latex particles have the highest charge amongthe aerosols tested. 4) Aerosols generated by mechanical dispersion have a charge higher than the Boltzmann charge. Introduction It is well known that most aerosol particles are more or less electrically charged. Some aerosols are charged during their generation and some are charged in the atmosphere by ions or in transportation by contact charging. Highly charged particles sometimes cause trouble in evaluating the performance of dust collectors or in evaluating aerosol behavior because of unexpected effects due to the electrical charge. The purpose of this paper is to provide a criterion for reference in selecting an aerosol for a certain experiment or in deciding whether or not a neutralizer is necessary. Several typical types of aerosol generators, familiar to aerosol researchers, have been tested for this purpose. Oneof the most familiar generators is the so-called evaporation-condensation type generator, which is knownas the La Mer-Sinclair generator, the Rapaport-Weinstock generator and so forth. In aerosol generators that use chemical reaction, the combustion type generator that uses, for example, burning tobacco or metals is often used because of its simplicity, and ammoniumchloride particles generated by chemical reaction are sometimes used for the same reason. Atomizing type aerosol generators including the vibrat- Received June 5, Correspondence concerning this article should be addressed to Y. Kousaka. M. Adachi is at the Radiation Center of Osaka Prefecture, Sakai 593 and K. Ebie is now with Shikishima Boseki Co., Ltd.. Osaka 541. ing orifice generator1] have been played an important role in aerosol research. Mechanical dispersion of powdery materials is useful for generating the large amountof aerosols that is often required to test dust collectors. The electric mobility distribution or the fraction of charged particles of these aerosols has been measured by the visual experimental technique in this work. The experimental results are compared with the Boltzmann equilibrium charge distribution to grvo a criterion of charge level of aerosol particles. 1. Experimental Apparatus and Method Figure 1 is a schematic diagram of the experimental apparatus for observation of the electric mobility of particles. Aerosol particles generated by a certain method are introduced into the observation cell through the copper pipe, having a diameter of 6 mm and a length of 1-3 m, which is connected to a ground line. The flow rate of aerosol through the pipe is //min. The effects of electrostatic diffusion and image force induced by charged aerosol particles on the deposition on the pipe wall are evaluated and found negligible6j7) by assuming that all particles are unipolarly and uniformly charged. After introducing an aerosol into the observation cell composed of vertical parallel-plate electrodes shown in Fig. 2, the aerosol flow is instantaneously stopped by closing the electro-magnetic valves. The aerosol particles inside 54 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

2 Table 1 Methods, particles and operating conditions of generator Method Aerosol Generating conditions Condensation Rapaport-Weinstock DOP nuclei : arc discharge (tungsten electrodes, nuclei flow after type generator rate=0.5 //min), carrier gas : nitrogen. 1.6 //min, evaporation evaporator temperature : 1 60 C La Mer-Sinclair DOPor Stearic nuclei : arc discharge (tungsten electrodes), nitrogen type generator acid flow rate= 1.6 //min, evaporator temperature : 170 C, reheater temperature : 250 C Chemical HC1 +NH4OH NH4C1 bubbling of hydrochloride gas in liquid ammonia, 1 //min reaction Combustion Tobacco and MgO air flow rate= 1 //min Drying after Atomizer and diffusion Di-octyl sebacate dried after atomization of isopropyl alcohol solution atomization dryer air flow rate= 1.6 //min NaCl or Polystyrene dried by silica gel after atomization of water solution latex or water suspension, air flow rate= 1.6 //min Mechanical Venturi CaCO3 feeding CaCO3powder into venturi throat, throat dispersion velocity= 100 m/sec, throat diameter of venturi = 1.2 mm, throat length= 5 mm,ratio of cross-sectional area at the throat to that of venturi inlet=0.15 (particle number concentration = particles/m3, polystyrene latex particles= 109 particles/m3) the cell then start to sedimentate by gravity. Whena certain voltage is applied to the electrodes, aerosol particles movetoward one of the electrodes according to the sign of the charged particles. As the focus of the microscope is set at a certain position inside the cell which is illuminated by a laser beam, the aerosol particles existing in the sensing volume of the microscope can be recognized because of light scattering. The movementsof particle images are firstly recorded by a video recorder. Reproducing the picture in slow motion, the horizontal velocity component, ue, of a particle between the electrodes is measured for each particle and is transformed to electric mobility by the relation Zp=ue/E. The measurement is repeated until the number of particles reaches about five hundred to obtain the electric mobility distribution. The intensity of electric field, E, ranges from V/m. The particle size distribution of the aerosols tested is measured separately by the ultramicroscopic technique5) after neutralizing them with an Am-241 a-ray source to remove the effects of electrostatic coagulation and diffusion. Table 1 shows the methods and the experimental conditions of various aerosol generators tested. 2. Experimental Results and Discussion 2. 1 Generation by condensation after evaporation La Mer-Sinclair4} and Rapaport-Weinstock3) generators are typical evaporation-condensation aerosol generators. The difference between them lies in the evaporation method: the former has a bath-type evaporator and the latter has an atomized droplet evaporator. Further, the latter is usually operated without supply of condensation nuclei, though the former usually needs nuclei. Fig. 1 Experimental apparatus for measurement of electric mobility of charged particles Fig. 2 Observation cell VOL. 14 NO

3 Fig. 3 Electric mobility distribution of DOP particles generated by Rapaport-Weinstock generator without nuclei Fig. 5 Electric mobility distribution of DOP particles generated by Rapaport-Weinstock generator with nuclei Fig. 4 Electric mobility distribution of DOP particles generated by La Mer-Sinclair generator Figures 3 and 4 show the results for DOPaerosols generated by the Rapaport-Weinstock and the La Mer-Sinclair generators respectively. In these figures, solid lines show electric mobility distributions of the aerosol particle at Boltzmann equilibrium, calculated from the particle size distributions separately measured. The Boltzmann charge distribution is given as follows : npln0 =Qxp (-p2e2/47re0dpfct) (1) It seems from these results that the aerosol particles generated by the La Mer-Sinclair generator are sufficiently less charged than the Boltzmann charge distribution. To see the effect of nuclei on particle charge, the Rapaport-Weinstock generator having condensation nuclei was tested. Ions generated by the arc discharge between two needles of tungsten were supplied as condensation nuclei. The result is shown in Fig. 5. Boltzmann distribution is almost attained in this case. Whenions as condensation nuclei were not supplied 56 Fig. 6 Electric mobility distribution of stearic acid particles generated by La Mer-Sinclair generator into the La Mer-Sinclair generator, DOP aerosol particles had no charge at all. It is considered that the higher charge of aerosol particles generated by the Rapaport-Weinstock generator compared to that by the La Mer-Sinclair generator is caused by electrical charging at the nebulizer. Figure 6 shows the electric mobility distribution at stearic acid aerosol particles generated by the La Mer-Sinclair generator under supply of nuclei. The electric charge of the stearic acid aerosol particles is less than that of DOPparticles generated by the same method Generation by chemical reaction The electric mobility distribution of magnesium oxide aerosol particles and tobacco smoke particles generated by combustion are shown in Figs. 7 and 8 respectively. The dotted line in Fig. 7 shows the experimental result when MgOparticles are neutralized by Am-241 a-ray radioactive source. Tobacco smoke particles shown in Fig. 8 seems to be not so highly charged as MgO aerosol particles. This difference JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

4 Fig. 7 Electric mobility distribution of magnesium oxide particles generated by combustion Fig. 9 Electric mobility distribution of sodium chloride particles dried after atomization Fig. 8 Electric mobility distribution of tobacco smokeparticles probably depends on the particle material, that is, metal or non-metal. Ammoniumchloride particles generated by bubbling chloride gas into ammoniaare found to be completely uncharged Generation by drying after atomization Most di-octyl sebacate particles dried by silica gel after atomization were uncharged, but sodium chloride particles generated by the same method have a higher charge than the Boltzmann charge distribution, as shown in Fig.9. Figure10 shows the result of polystyrene latex particles dried after atomization. As is seen in the figure, the particles are highly charged. The average charge of both negative and positive particles in this case corresponds to several handred elementary charge units per particle, which is the highest value among the aerosols tested. Whenaerosols have unipolar charge and high concentration, there is a limit in measuring electrical mobility because of the electric field induced by space charge. Even if all particles tested are charged in VOL 14 NO Fig. 10 Electric mobility distribution of polystyrene latex particles dried after atomization the same sign, in unipolarity, the electric field induced by the space charge is less than 10% of that applied to the electrodes of the cell. It is suggested from these results that charge quantity of aerosol particles dried after atomization greatly depends on the state of solution or suspension Generation by mechanical dispersion Figure ll shows the electric mobility distribution of calcium carbonate aerosol particles dispersed by a jet stream at the throat of a venturi madeof glass. The dotted line shows the experimental result when particles are neutralized by Am-241. The fine and the thick solid lines show the distributions calculated under assumption of the Boltzmann equilibrium from the particle size distributions separately measured. The fine line in the figure is calculated from the Stokes diameter based on the bulk density of aggregates in a packed bed and the thick line is based on the primary particle density. Since the charging mechanism of aggregate particles has, so far, not been made clear, these lines are only for reference in this study. Further discussion will be necessary for aggregate particles. 57

5 Fig. ll Electric mobility distribution of calcium carbonate particles mechanically dispersed by an air stream at venturi Fig. 12 Numberfraction of charged particle It is necessary to neutralize aggregate particles generated by mechanical dispersion to decrease charge level Number fraction of charged particle The number fraction of charged particles, which is expressed by the ratio of number concentration of charged particle to total number concentration, is shown in Fig. 12 together with The tendency of the results Dallavalle's obtained in results2k this study agrees with Dallavalle's results. carrying highest charge are The aerosol also found to particles be polystyrene latex dried after atomization. Figure 12 is convenient as a reference concerning the charge level of various particles generated by various methods. Conclusion The results obtained in this study are as follows. 1) Aerosol particles generated by condensation after evaporation almost reach Boltzmann charge distribution when ions are mixed as condensation nuclei. On the other hand, they have a lower charge when no ions are mixed. 2) The charge quantity of aerosol particles generated by chemical reaction depends on the particle material and the kinds of reaction. The MgOaerosol particles generated by combustion are highly charged, but tobacco smoke particles are not highly charged. Ammoniumchloride particles generated by chemical reaction are not charged at all. 3) The charge quantity of aerosols generated by drying after atomization seems to be affected by the state of solution or suspension in nebulizer: di-octyl sebacate particles are almost uncharged, sodium chloride particles are highly charged, and polystyrene latex particles are very highly charged. 4) Aerosol particles generated by mechanical dispersion have a charge higher than the Boltzmann charge. To make clear the charging mechanism of aerosol particles is important, and this paper is the first step in an approach to makethis charging mechanism clear. Nomenclature dp = particle diameter I>m] E = intensity of electric field [V/m] e = elementary electric charge (= x lo"19) [C] /(In \ZP\) = frequency of particle number concentration in an electrical mobility band (=An\ntA In \ZV\) [-] n = particle numberconcentration [particles/m3] nt = total particle numberconcentration [particles/m3] p = number of elementary charge [-] p&r = average of elementary charge [-] T = absolute temperature [K] ue = velocity of particle toward electric field [m/sec] Zv = electrical mobility (=ue/e) [m2/v sec] A \n \Zp\ = electrical mobility band [-] An = particle numberconcentration in an electrical mobility band [particles/m3] 0 = dielectric constant (=8.855 x 10~12) [F/m] k = Boltzmann's constant (=1.3806X10~23) [J/K] og = geometric standard deviation [-] <Subscripts> p 0 - p elementary charged = uncharged + = positive - =negative Literature Cited 1) Berglund, R. N. and B. Y. Liu: Environ. Sci. TechnoL, 7, 147 (1973). 2) Dallavalle, J. M., C. Orr and B. L. Hinkle: Brit. J. Appl Phys., suppl, 3, 198 (1954). 3) Rapaport, E. and S. E. Weinstock: Experientia, ll, 363 (1955). 4) Sinclair,D. and V.K. La Mer: Chem. Rev., 44, 245 (1949). 5) Yoshida, T., Y.Kousaka and K. Okuyama: Ind. Eng. Chem., Fundam., 14, 47 (1975). 6) Yu, C. P.: /. AerosolSci., 8, 237 (1977). 7) Zebel, G.: "Aerosol Science", ed. C.N. Davies, p.42, Academic Press, London and New York (1966). 58 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

Generation of monodisperse aerosols through condensation nuclei control

Air Pollution XV 505 Generation of monodisperse aerosols through condensation nuclei control H. M. Kadlimatti 1, S. Gangamma 2 & S. K. Varghese 3 1 Department of Mechanical Engineering, Basaveshwar Engineering