An Aerosol Generator for High Concentrations of µm Solid Particles of Practical Monodispersity

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: An Aerosol Generator for High Concentrations of µm Solid Particles of Practical Monodispersity D. A. Japuntich, J. I. T. Stenhouse & B. Y. H. Liu To cite this article: D. A. Japuntich, J. I. T. Stenhouse & B. Y. H. Liu (1992) An Aerosol Generator for High Concentrations of µm Solid Particles of Practical Monodispersity, Aerosol Science and Technology, 16:4, , DOI: / To link to this article: Published online: 11 Jun Submit your article to this journal Article views: 262 View related articles Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 21 December 2017, At: 16:41

2 An Aerosol Generator for High Concentrations of pm Solid Particles of Practical Monodispersity D. A. ~apuntich* Occupational Health and Environmental Safety Division, 3M Company, Building 260-3B-08, 3M Center, St. Paul, MN J. I. T. Stenhouse Department of Chemical Engineering, Loughborough University of Technology, Loughborough, Leicestershire, UK LE113TU B. Y. H. Liu Department of Mechanical Engineering, Uninversity of Minnesota, 125 Mechanical Engineering, Ill Church Street, S.E., Minneapolis, MN A continuous-flow, evaporation-condensation aerosol generator has been designed to produce particles of practical monodispersity of stearic acid in concentrations of over 1 g/m3 at flow rates > 6 L/min. Pure stearic acid containing a dissolved impurity is meltsprayed and evaporated, producing a nuclei- vapor mixture. The mixture is recondensed and then quickly quenched into spherical, solid particles of a narrow size distribution. The condenser design is a straight, insulated glass tube of 5 cm in inner diameter and of 110 cm in length. A heating and flow straightening conditioning section previous to the condenser provides a relatively flat condensation front across the tube diameter, while the insulated condenser walls in free convection create a low radial temperature gradient, both of which enhance particle monodispersity with particle geometric standard deviations < The dynamic condenser conditions for the suppression of homogeneous nucleation were investigated as a function of the ratio of the Grashof-Prandtl numbers product to the Reynolds number. INTRODUCTION Continuous-flow heterogeneous condensation aerosol generators are used to create near-monodisperse particles for lung inhalation deposition studies, air filter testing, or as processes to manufacture high-purity ma- *To whom correspondence should be addressed. terials from vapor-phase reactions. Examples of such aerosol generators are the Sinclair-LaMer generator (Sinclair and LaMer, l949), the Prodi generator (Prodi, 1972), the falling film generator (Nicolaon et al., l97o), the Tu single stage generator (Tu, 1982), the Kogan-Burnasheva generator (Kogan and Burnasheva, 1960), and the Rapaport and Weinstock generator (Rapa- Aerosol Science and Technology 16: (1992) O 1992 Elsevier Science Publishing Co., Inc.

3 Monodisperse Aerosol Generator for pm Solid Particles 247 port and Weinstock, 1955). An important goal is the stipulation of practical monodispersity, defined as a particle size distribution with a geometric standard deviation (GSD) < 1.25, as defined in Fuchs and Sutugin (1966). A comprehensive investigation into aerosol formation by homogeneous and heterogeneous nucleation using a laminar continuous-flow aerosol generator was presented by Nguyen et al. (1987), including comparisons to the model of Pesthy et al. (1983). A review of the condenser conditions for practical monodispersity of many examples of these aerosol generators using dimensionless heat and mass transfer groups was given in Japuntich et al. (1990). The objective of this study was to design and optimize a solid particle aerosol generator which could produce discrete particle diameters of 1-5 pm at high concentration output rates of 10 mg/min for use in air filter loading experiments. This application required a single monodisperse distribution. No other secondary distributions or spurious particles could be tolerated. A review by Japuntich (1991), limited to organic condensates, showed that present evaporation-condensation generators could reach only a maximum aerosol mass concentration output of 1.5 mg/min at no greater than 3 L/rnin. Particle diameters reported > 1.8 pm, sometimes shown with the Sinclair-LaMer generator (Bowes, 1986), were very rare, and it became evident that a new scheme was necessary to meet the design constraints. AEROSOL GENERATOR The Rapaport and Weinstock (1955) aerosol generator was chosen as the archetype for the aerosol generator design, in that the nuclei production, vapor generation, and vapor-nuclei mixing are all included in the nebulizing section. Liu et al. (1966), Tomaides et al. (1971), and Lee and Liu (1975) greatly refined this generator to make its operaton with time more stable and to improve the ease of selection of different particle diameters. The result of this research is the aerosol generator shown in Figure 1. The entire generator is shielded from changes in the room temperature by placement in an oven at constant conditions, hot enough to maintain the normally solid condensate-impurity solution in a liquid state so that it may be nebulized into droplets with a nitrogen carrier gas. These droplets are evaporated, leaving behind impurity nuclei which are hot-filtered for a vapor -nuclei concentration ratio choice of particle diameter, condensed into particles in a special temperature and flow field, and then quenched into a solid particle aerosol challenge with cool, clean dilution air. Nuclei filter changes may be taken as a gross particle size adjustment. This provides a method of obtaining logarithmic increments of particle diameter in the range of pm in aerodynamic diameter. Fine adjustments to the diameter may be made by altering the diffusion capture mechanisms of the nuclei filter penetration by changing either the nitrogen flow (changes filter face velocity) or by changing the percentage of impurity in the melt (changes nuclei diameter). In this way, particle diameter adjustments to within a tenth of an aerodynamic particle diameter may be made, and any diameter drift during filter testing may be corrected. The system takes about 1 h to warm up from a cold start, but if the oven and heaters are left on, the time it takes for a useful, stable aerosol is about 30 min after the nebulizer is turned on. After the equilibrium has taken place, diameter stabilization after a filter change takes < 15 min. The generator has been operated with only slight adjustments to give solid stearic acid particles for > 6 h at 4.0 pm in particle diameter with GSD < In these studies the output particle size distributions were measured after dilution by using an Aerodynamic Par-

4 248 Japuntich et al. MOVABLE HOT PLATE: FOR FILTER CHANGIN NUCLEI Fl LTRATION UCLEI FILTER ELECTRICAL TERMINALS HEATING AND CONDIT1ONING HEATING TAPE ALUMINIUM CORE FLOW STRAIGHTENERS CONDENSER COOL DILUTION + I TO TEST DUCT AIR FIGURE 1. Solid particle continuous-flow condensation aerosol generator.

5 Monodisperse Aerosol Generator for pm Solid Particles 249 ticle Sizer (APS), Model APS-3310 with the Advanced APS Software, Model ( pm range), and a Differential Mobility Size Analyzer (DMPS) with its statistical software package ( pm range). This equipment is manufactured by TSI, Inc., St. Paul, MN. CONDENSATE MATERIALS Common solid organic materials of great purity were investigated with the needs of low melting points and low vapor diffusion coefficients. Two solid condensate materials were chosen, stearic acid and cholesterol, with specific gravities of 0.85 and 1.067, respectively. After much experimentation showing excellent initial monodispersity, it later became apparent that the cholesterol was unsuitable for long-term use, because even in nitrogen it degrades at temperatures around its boiling point (633 OK). Stearic acid is readily available in a very pure form and is nontoxic, but has a low thermal conductivity and needs to be chilled quickly in droplet form or the particles will grow into nonspherical crystalline structures. The dilution air in this system was found to be adequate to freeze the particles into amorphous spheres which were more than satisfactory for filter loading test times. However, when particles were sampled onto microscope slides, after 12 h or longer the spheres began to crystalize into nonspherical particles at room temperature. The impurity material for the nuclei was chosen to be Irganox 1010, an antioxidant used to stabilize polymers during extrusion and for retarding thermal degradation in oils used in fat frying. The melt solution mass ratio of Irganox 1010 to the condensate gives the nuclei particle diameter (d,,,,,,) if the nebulizer spray particle mean diameter is measured and the nuclei material density (p,) is known. For a spray particle diameter of 0.4 pm, a mixture mass concentration ratio of 0.032% gives an acceptable predicted mean nuclei diameter of pm, smaller than the mean free path of nitrogen, pm, at an average condenser tube centerline temperature from the following equation: mass of nuclei material ( mass of condensate material It should be noted that other popular "liquid" condensates have worked well in this generator. For example, dioctyl phthalate containing Irganox 1010 has given particle diameters > 5 pm with GSD < 1.20 at stearic acid operating conditions. VAPOR-NUCLEI GENERATION The choice of a nebulizer was a classic six-jet Collison nebulizer (British Standard 2984). Its glass and metal design allowed it to withstand high temperatures, and the replaceable glass jar allowed for quick refills of condensate solution. The mass concentration outputs achievable at higher temperatures for this nebulizer are remarkable for organic liquids when compared to room temperature. This is attributable to great increases in the recirculation of the melt liquid within the nebulizer as a result of low liquid viscosities at high temperatures. A cylindrical impactor of mm inner diameter was fitted over the spray head with the objectives of reducing the mass concentration and to reduce the size range and the GSD of the spray particles and consequently the condensation nuclei. This helps to improve the monodispersity of the condensate particles. The output of the nebulizer-impactor system at 365 K oven temperature for a stearic acid spray particle size distribution of 0.46 geometric mean diameter (GMD) and 1.50 GSD was well over 2500 mg/m3 at 6 L/min (15 mg/min). Interestingly, the normally

6 250 Japuntich et al. encountered reduction in nebulizer output concentration with a reduction in liquid level (as much as a centimeter) was not a problem. VARYING THE PARTICLE DIAMETER The spray-carrier gas mixture passes through the evaporator (an insulated 2.54-cm diameter, 58-cm-long glass tube covered with heating tape run at 235 W), becoming a nuclei-vapor mixture challenge to the hot nuclei filter. The hot nuclei filter material used in different numbers of layers for different particle diameter selections was AF- 11 Plain fiberglass filter material made by Manville, Defiance, OH. AF-11 is a mat filter composed of fiber diameters of about 5 pm at a basis weight of 62 g/m2. It was compressed and contained between two stainless steel screens, molded into flexible cylinders with face areas of 44 cm2. Filters were clamped into the filter housing by a hot plate mounted on a moving air cylinder for easy replacement. As shown in Swift (1967), the particle size output of condensation generators is dictated by the ratio of the condensation nuclei and vapor concentrations: where d, is the condensed particle diameter (m), co is the vapor concentration (g/m3), p, is the condensate density (g/m3), and n is the nuclei concentration (number/m3). In reality, there is always condensation wall loss of vapor, the possibility of homogeneous condensates and the thermophoretic loss of nuclei to the condenser walls to complicate this equation. The generator nuclei concentration to give 5-pm particles at the measured nebulizer mass concentration output was estimated by Eq. 2 to be 5 x lo4 particles/cm3, well above the lo3 particles/cm3 approximate limit below which Eq. 2 is no longer valid as found in Swift (1967), Kogan and Burnasheva (1960), and Nguyen et al. (1987). The classical theoretical exponential expression for changes in filter penetration with filter structure, in this case number of filter layers, is shown in Eq. 3: "'2 Pn = EXP ( - k (filter layers) } = - (3) n1 where Pn is the filter penetration, n, and n, are the nuclei concentrations upstream and downstream to the filter, respectively, and k is a constant of the filter structure and nuclei particle capture performance (which may be changed slightly with nuclei diameter and carrier gas flow). The use of filters to change the nucleivapor relationship and the subsequent particle size can be derived from the ratio of Equation 2 at two selected particle diameters, giving: Combining Eq. 3 and 4 gives the following general form for particle diameter change with number of layers of a filter medium: d, = ~,,EXP { ( ) (filter layers) } (5) For the AF-11 filter medium at 7 L/min nitrogen volume flow, the Eq. 5 relationship was verified experimentally in Figure 2 to be an exponential equation for stearic acid with a base diameter of 0.69 pm according to the following regression with RSQ = 0.99: dp2 = 0.69 EXP (0.205 (filter layers) } (6) As the nuclei concentration is decreased by filtration to grow larger particles, the mass concentration decreases to some extent due to vapor wall loss. As an added note, as the carrier gas flow is decreased, the GSD decreases and the particles tend to grow slightly larger. Lowering the mass concentration by selectively plugging the liquid feed holes in

7 Monodisperse Aerosol Generator for pm Solid Particles J NUMBER OF NUCLEI FILTER LAYERS FIGURE 2. Aerodynamic geometric mean diameter (pm) versus number of nuclei filter layers (filter penetration in parentheses). the Collison nebulizer also improves the monodispersity. CONDENSER EFFECTS ON SIZE AND MONODISPERSITY In the research leading to this design, many condenser tube strategies were tried. In general, tubes smaller than the 3-cm diameter gave particle diameter maximums < 3-pm and low concentrations. The 1.8-pm maximum for most of the generators in the literature (Japuntich, 1991) seems to be an artifact of the popular 2.54-cm original Sinclair-LaMer generator tube diameter. Condensation in glass tubes showed a clear area next to the condenser wall caused by thermophoretic loss of nuclei, resulting in local vapor wall loss. The thickness of this area did not change with tube diameter. In order to maximize both particle diameter and mass concentration, the 5-cm diameter, 110-cm-long glass tube condenser in Figure 1 was chosen after Tu (1982), nearly twice the diameter of the Sinclair-LaMer generator. A larger condenser tube mini- mizes the thermophoretic wall loss of nuclei and vapor' wall loss by a decrease in the ratio of the volume flow rate to wall area. The addition of a heated flow straightening section as a flow field and temperature field conditioner created a relatively flat condensation front across the tube diameter, greatly enhancing particle monodispersity. The three aluminum core flow straighteners were of 0.32-cm cell diameter by 5.8-cm thickness, spaced 7.6 cm apart. The first 36 cm of the tube was covered with heating tape run at 36 W for a typical oven temperature of 365 K for stearic acid. The use of free convection cooling and the addition of insulated walls (7-,urn diameter fiberglass, 750 mg/m2), which was also used by Tu (1982), gave particle output GSD of < 1.23, as compared to 1.35 for forced convection wall cooling experiments. Typical measured centerline and wall temperatures (+ 1 K) down the tube during condensation at the optimized conditions for stearic acid (GSD < 1.24) are shown in Japuntich (1991). At the optimization conditions for stearic acid, the inlet temperature

8 252 Japuntich et al. (To) at 442 K and oven temperature at 390 K gave a tube centerline to wall difference of 36 K at the condenser inlet, an average temperature difference in the tube of 21 OK, and an exit temperature of 404 OK. At the high vapor concentrations used in this generator, supersaturation conditions may exist for the formation of nuisance homogeneous condensates. With this in mind, the conditions for the suppression of homogeneous condensates were investigated. The generator aerosol output was measured simultaneously after careful dilution with the APS and the DMPS, covering a range 0.01 to 30 pm. When suitable nuclei filters were installed and the heterogeneous condensate monodisperse diameter was increased to > 3.5 pm by decreasing the nuclei to vapor concentration ratio, small homogeneous condensate particles < 0.15 pm could be created by elevating the carrier gas volume flow. This gave two particle size distributions: a monodisperse heterogeneous condensate distribution (as measured by the APS) and a polydisperse homogeneous con- densate distribution (as measured by the DMPS). Figure 3 shows how the GMD of the two distributions as calculated by the TSI software packages vary with increasing flow. The small homogeneous condensate particles at 10 L/min nitrogen volume flow, which at < 0.1 pm are below the detection limits of the APS, were found to be as much as 10% of the total mass concentration of the total generator output of 12 mglmin. Such results are not atypical of other condensation aerosol generators, and researchers are cautioned to always measure a wide particle diameter spectrum before reporting monodispersity results, specifically examining the pm region where homogeneous condensates usually form. A more general study was necessary for the optimization of the system since the condensation conditions are affected by the heat transfer conditons as well as the nitrogen carrier gas flow. The shape of the condensation profile is affected by the balance of the laminar forced convection forces down COLD NITROGEN NEBULIZER FLOW (Ipm) FIGURE 3. Heterogeneous and homogeneous geometric mean particle diameters versus cold nitrogen nebulizer volume flow (L /min).

9 Monodisperse Aerosol Generator for pm Solid Particles 253 the tube and the free convection buoyancy forces in the tube, as described in Japuntich et al. (1990). The product of the Grashof- Prantl numbers (GrPr) describes the free convection "buoyancy effect" on the flow field, GrPr = gp D~AT/ v a, where g is the gravitational constant (m/s2), /3 is the coefficient of thermal expansion (K- I), D is the condenser tube diameter (m), AT is the condenser inlet to wall temperature difference (K), v is the carrier gas kinematic viscosity (m2/s), and a is the carrier gas thermal difisivity (m2/s). The Reynolds number describes the forced convection conditions down the tube; Re = DUO /2v, where Uo is the average gas velocity (m/s) at the average condenser temperature. The ratio of the GrPr product to Re gives an estimate of the proper balance of the convection conditions in the tube for homogenous condensate suppression. A more detailed experiment varying the forced and free convection conditions was run at different nitrogen volume flows at two oven temperatures, giving different condenser wall temperature profiles down the tube. At each condition calculations were made of the ratio of the mass concentrations. 366 K OVEN. 394 K OVEN at the GMD of both the heterogeneous and the homogeneous distributions. This ratio was plotted against the GrPrlRe ratio, calculated at the measured average wall and inlet temperatures. Figure 4 shows that conditions may found where the homogeneous condensates could be suppressed by varying either the flow field (carrier gas volume flow) and/or the heat transfer conditions (wall temperature). In general, if the GrPrlRe ratio is > 1000 for this generator using stearic acid and a nitrogen carrier gas, only the large heterogenous condensate distribution will exist, giving mass ratios well above 250. The following table shows typical aerodynamic GMD and GSD run at the optimized conditions as measured after careful dilution with the APS: GMD(pm) GSD (pm) SUMMARY AND CONCLUSIONS An aerosol generator capable of producing solid particles of practical monodispersity of 1-5 pm in diameter has been designed, FIGURE 4. GrPrlRe versus mass ratio of heterogeneous to homogeneous condensate particles.

10 254 Japuntich et al. fabricated, and optimized. This design provides a mass concentration output six times greater than for other generators of this type. A stearic acid mixture with Irganox 1010 antioxidant as the nuclei source gave excellent stability results. Other pure condensate substances of high molecular weight with low vapor pressures, such as dioctyl phthalate, may also be used with success. The use of hot filtration of the nucleivapor mixture has been shown to be a good method of particle range size selection. Fine tuning of the particle size may be done within certain limits by changing the nitrogen flow or by changing the percent nuclei material in the condensate melt in the nebulizer. Dimensionless parameters for condenser conditions for monodispersity have been successfully applied to the conditions for the monodispersity of the particle size distribution, specifically the suppression of nuisance homogeneous condensates. REFERENCES Bowes, S. M. (1986). Particle Deposition In The Mouth During Oro-Nasal Breathing. Ph. D. Thesis. Johns Hopkins University, Baltimore, M. D. Fuchs, N. A,, and Sutugin, A. G. (1966). In Aerosol Science, (C. N. Davies, ed. Academic Press, London, pp Japuntich, D. A. (1991). Particle Clogging of Fibrous Filters. Ph. D. Thesis. Chemical Engineering Department, Loughborough University of Technology, Loughborough, UK. Japuntich, D. A,, Stenhouse, J. I. T., and Liu B. Y. H. (1990). J. Colloid Interface Sci. 136: Liu, B. Y. H., and Lee, K. W. (1975). Am. Ind. Hyg. Assoc. J. 36: Liu, B. Y. H., Whitby, K. T., and Yu, H. H. S. (1966). J. Rech. Atmos. 3: Kogan, Y. I., and Burnasheva, Z. A. (1960). Russ. J. Phys. Chem. 34: Nguyen, H. V., Okuyama, K., Mimura, T., Kousaki, Y., Flagan, R. C., and Seinfeld, J. H. (1987). J. Colloid Interface Sci. 119: Nicolaon, G. D., Cooke, D. D., Kerker, M., and Matijevic, E. (1970). J. Colloid Interface Sci. 34: Pesthy, A. J., Flagan, R. C., and Seinfield, J. H. (1983). J. Colloid Interface Sci. 91: Prodi, V. (1972). In Assessment of Airborne Particles (T. T. Mercer, P. E. Morrow, and W. Stober, eds.). C. C. Thomas, Springfield, IL, p Rapaport, E., and Weinstock, S. G. (1955). Experimentia 11:363. Sinclair, D., and LaMer, V. K. (1949). Chem. Rev. 44: Swift, D. L. (1967). Ann. Occup. Hyg. 10: Tomaides, M., Liu, B. Y. H., and Whitby, K. T. (1971). J. Aerosol Sci. 2: Tu, K. (1982). J. Aerosol Sci. 13: Received September 25, 1991; accepted January 14, 1992.