-4 group ,of papers from
the Symgosiam on Air Pollution, Division of Analytical Chemistry, 130th Meeting of American Chemical Society, Atlantic City, N. J., September 1956
AIR POLLUTION ~~
~~~~~~~
~
Critical Comparison of Collection Efficiencies of Commonly Used Aerosol Sampling Devices CONRAD SCHADT and R. D CADLE Stanford Research Institute, Menlo Park, Calif.
b The extent to which the theory of particle collection techniques could b e applied to commonly used field instruments has been determined. The instruments included sedimentation chambers, an MSA electric precipitator, a Greenburg-Smith impinger, Millipore filters, the Cassella thermal precipitator, and an impactor. The aerosols used for determining efficiency were nearly monodisperse-that is, the particles were nearly all the same size. The use of such aerosols greatly simplifies the problem of comparing the efficiencies of the instruments for collecting particles of different sizes. Perhaps the most interesting feature of the results obtained has been the marked discrepancies often observed between theoretical and observed efficiencies. Possible reasons for these discrepancies are discussed.
M.
smokes, or dusts are of great importance today in connection with air pollution problems, recovery of valuable materials otherwise lost from stacks, and in processes requiring the formation of aerosols, such as spray painting. The need to study these various aerosols has resulted in the use of many sampling methods and devices to collect aerosol particles for measurements of mass, particle size distributions, and other physical and chemical properties. The collection efficiencies of the various sampling devices often vary greatly for different aerosols and different conditions of operation. This makes the selection of sampling devices for .4SY AEROSOLS,
864
ANALYTICAL CHEMISTRY
any particular laboratory or field program of aerosol studies a difficult problem. Fortunately, most of the numerous devices in use can be classified according to a few basic principles of operation. The purpose of the experimental program on n-hich this paper is based is to present information on each of these types of devices which mill serve as a guide for the selection of sampling instruments for various field or laboratory programs. The following sampling devices were used in this study: Thermal precipitator Jet impactor Greenburg-Smith impinger Electrostatic precipitator Membrane filter i3lillipore filtrr) Sedimentation box EXPERIMENTAL
Equipment. The thermal precipitator wa5 a modified commercial unit, manufactured by C.F. Casella Br Go., Ltd., London, Enqland. The usual mire heating filament was replaced for these studies with a ribbon filament. The aerosol was drawn past the hot filament by allowing water to flow from a tank to which the precipitator head is attached. The particles nere collccted on microscope cover slides locatrd on opposite sides of the hot filament Important dimensions of the precipitator are : Filament width 1 . 4 nim. Length of exposed filament (normal to direction of air AOW)
Distance from filament to each cover slide
10
mm.
0 15mm.
Temperature gradients n ere deter-
mined using a copper-constantan thermocouple made from 1-mil Fire. The jet impactor consisted of a single rectangular jet opposite a microscopeslide collecting surface. The jet mas mounted in a metal housing which had a track to hold the microscope slide a t the proper distance from the jet. Air n-as drawn through the jet by applying a vacuum to the metal housing. The instrument had the folloning dimensions: Jet length 11 mm. 0 . 2 0 mm. Jet n-idt'h Distance from jet' to collect0.53 mm. ing surface This impactor is not commercially available, but is similar to a single stage of the cascade impactor manufactured by C. F. Casella R- Co., Lt'd., London, England. The Greenburg-Smith inipinger (4) wis constructed in the glass shop of Stanford Research Instit,ute. The follo\Ting are the dimensions of the unit used in the tests: Diameter of collecting tube 60 nim. Jet diameter 2 3mm. Distance from jet t o flat hottom
Liquid level
5 50
mm.
mni.
The electrostatic precipitator was an 3ISA Electrostatic Sampler, Model F , made by Mine Safety lppliances Co., Pittsburgh, Pa. Membrane filters . 3 ) are commercially available from the 1Iillipore Filter Corp., Waterton-n, Mass. Two types of Millipore filters were t'ested, AA (aerosol assay) and HA (hydrosol assay). A sedinientation box was constructed of Styrafoam (for thermal insulation) and the interior was lined with sheet aluminum (for an electrically conducting surface), This surface was electrically
ator previously described (1). Size distributions of the aerosol particles were determined from samples collected with a Casella cascade impactor. The size of the glycerol droplets was varied by varying the concentration of the glycerol solution. Carbonyl iron was obtained from the General Dyestuff Corp. Chemical analysis showed that it contained 9i.375 iron. The iron dust was dispersed with a n air nozzle designed for use with powders (1). A single particle size distribution was used, which was determined by microscopic examination of the collected particles (Figure 1). Test Procedure. A typical run with a thermal precipitator consisted of generating the aerosol, draning it through the precipitator using a predetermined flow rate and thermal gradient, and measuring the widths of the deposits of the t n o coyer slides. The other sampling instruments were tested in a 10-cubic meter test chamber in which aerosols were formed. The sedimentation box Tvas opened and aerosol was gently blown into the box with a fan operated a t very low speed. The lid was closed and the particles were allowed t o settle 8 to 24 hours, depending on particle size. Microscopic counts were then made and concentrations calculated from these data and from the volunie of the box. The jet impactor, Greenburg-Smith impinger, electrostatic precipitator, and membrane filter were each tested in the following manner. Aerosol was drawn through the device a t a predetermined flow rate. The aerosol which passei through was sampled by an aerosol photometer, which compared this aercsol concentration with that in the test chamber. These measurements gave the relative amount of aerosol which passed through each sampling device Some additional runs were made by placing Millipore filters in series with other devices to collect the particles passed. Direct Millipore filter samples were also taken and microscope counts were made on both sets. Comparison of these counts gave the relative number of particles passed by each sampling device.
grounded during experiments. The top of the box n-as hinged so t h a t it could be opened for rapid filling with aerosol and so that collecting slides could be readily inserted or removed. Microscope slides with and without electrically conducting layers of Aquadag (dcheson Colloids Co., Port Huron, Afich.) were used as collecting surfaces for subsequent microscopic examination. The inside dimensions of the box were 25.4 em. high and 35.6 X 40.6 cm. cross section. TITOaerosol photometers were used to measure relative aerosol concentrations: a n NRL Smoke Penetration Meter, on loan from the Chemical Corps, and a Sinclair-Phoenix forwardscattering smoke photometer. The XRL instrument u-as used for most of the measurements, as it \vas the only one available a t first; and the SinclairPhoenix was used for the most recent tests, when it was the only one available. A cascade impactor made by C. F. Casella & Co., Ltd., was used with the latter photometer to provide inforniation on part’icle size of glycerol aerosols. Particle Preparation. The stearic arid (Eastnian Kodak Co.) was redistilled before use. Aerosols of stcwic acid were prepared using a La Mer condensation-type generator ( I O ) . Part’icle sizes were determined using t’he “01~1” (Q) and microscopic mamination of collected particles. Nost of these aerosols exhibited higher order Tyndall spectra, indicating that they were nearly monodisperse. The sodium chloride \vas reagent grade and was not further purified. Aqueous solutions were dispersed using an aspiratortype aerosol generator similar to that described by Cadle and Magill ( 1 ) . The newly formed aerosols were passed through a bed of glass beads to remove the larger particles. Evaporation of the water droplets left an aerosol of sodium chloride crystals. Size distributions of the aerosol particles mere determined by preparing elect,ron micrographs of particles collected with the thermal precipitator. The size of the suspended salt crystals was varied by varying the concentration of the salt solution. A typical size distribution is shown in Figure 1. Reagent grade glycerol was used to prepare glycerol aerosols. Aqueous solutions were dispersed in the same manner as the sodium chloride solutions, using the aspirator-type aerosol gener-
Y
z 0
RESULTS A N D DISCUSSION
Thermal Precipitator. Epstein has developed an extension of radiometer
SODILIV CHLOR DE
CARBONYL
IRON
30
20
IO
PARTICLE
Figure 1,
8 ‘0 0.5 1.0 1.5 DIAMETER MICRONS
-
2.0
Particle size distributions
2.5
3.0 3.5
4.0
4.5
theory which is generally considered the best explanation of thermal force (2). This theory considers adsorption and desorption of gas molecules and “thermal creep” of the molecules along the surface of the particle in the thermal gradient. The thermal gradient and the thermal conductivities of the particle and of the gas surrounding it determine the temperature distribution in the particle. Thermal force is given by
Good agreement with this theory n-as obtained by Rosenblatt and La Rfer ( 6 ) using tricresyl phosphate, and by Saxton and Ranz ( 7 ) using paraffin oil and castor oil. All these materials have fairly low thermal conductivities (Table I) Equation 1 shows that for materials with large thermal conductivities the thermal force would be small. One might therefore expect to experience difficulty in collecting materials vith high thermal conductivities by the method of thermal precipitation. Such difficulty has not been reported in the literature although this instrument has been in use for many years. Therefore, experiments m-ere performed with materials covering a wide range of thermal conductivities (Table I). I
Table 1.
Thermal Conductivities
Aerosol
AIaterial Air Stearic acid Glycerol Sodium chloride Iron Tricresyl phosphate Castor oil Paraffin oil
Thermal Conductivity, Cal./Cm. Sec. ’ I
