Electrostatic Sampler for Dust-laden Gases G. L. ROUNDS and H. 1. MAT01 Air Control and Research, Kaiser Steel Corp., Fontana, Calif sampling. This equipmcnt was designed in 1953 and subsequent testing has shown it to be applicable to requirements A and C and, providing appropriate insulators are used, to D and E. The Mine Safety Appliances Co., Pittsburgh, Pa., has an electrost,atic sampler which is designed for sampling the atmosphere, but makes no provision for sampling stzck gases. The Western Precipitation Corp., Lo8 Angeles, Calif., manufactures an eleetrostatic stack eas samvler called the “sampling electrofilter.” €Iowever, this unit incorporates a cylindrical collecting tube and is thus restricted to a dust-loading limit of from 1 to 2 grams. Furthermore, the Western unit incorporates a porcelain insulator wbich is subject to attack by open hearth gases. The electrostatic sampler designed by the authors has a very low pressure drop a t the rated Row of 1.5 cubic feet per minute or less. The device is constructed of stainless steel, which minimisea reaction with the stack gases and allows a wide range in sampling temperatures. The collected dust sample can be quantitativel. recovered from the inside walls of the sampling tube with no contamination from the sampler. As a power source, the standard Mine Safety Appliance “power pack” used on the M.S.A. electrostatic sampler has been found very reliable. Using this murce, it is possible to obtain approximately 15 kv. across the precipitat.or, depending on the relative humidity of the gzses being sampled and the electrical properties of the dust being collected. Fluctuations in the llOvolt pox-er source cause significant variations in the output kilovoltsge, which necessitate fairly frequent observat.ion and readjustment. of the poser pack output. All of the stack gases and atmospheric samples tested, to date, have been quantitatively cleaned of the particulate material hy 13 kv. impressed across the precipitator. This potential also allatis Some leeway for increasing the output. in the event of a drop in t.he 110-volt power murce. Figure 1 portrays the sampling setup, including B pair of plastic impingers designed by the authors, and Figure8 2 and 3 show bhe construction of the sampler.
An electrostatic sampler has heen designed, which will collect up to five t i m e s the r e i g h t of dust collected by oonventional electrostatic samplers. The device uses a eommareially available h i g h voltage source. The eollecting surface is i n the form of a t r u n c a t e d oone for more uniform dust deposition. T h e u n i t can he disassembled i n a few seconds for cleaning.
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X T H E process of accurately sampling a. gaseous mixture for both gaseous and solid constituents, one of the most. important considerations is the selection of the device that nil1 provide the initial separation of the gases and the solids. This selection is dependent upon the suhsequent analytical treatment to which the sample is to be subjected. The follotiing treatments and requirements are considered important. in the field of air pollution. Dust loading of gas stream. A highly efficientdevice. B. Chemical determination of the elements in their native state of oxidation in the gsSeous and solid phases. A device that subjects the sample to neither an ovidieing nor reducing atmosphere. C. Complete ohemioal analyses of the two phases. A device that will not contaminate sample or react with it. D. Analyses oi the gaseous portion for compounds likely to adsorb on the surface of the partieulat,e. A devioe capable of performing at the temperatures existing in the native gassolid mixture. and which provides minimum contact between the gaseou8 and solid phases. E. Chemioal determination of elements or compounds that occur in both phases. A highly efficient device, which provides minimum contact between the two phases, and is oapable of performing at the temperatures existing in the native gatsolid mixture. F. Partiole siae determination. A nonagelomerating. nonfreeturing, and highly efficient devioe. A.
The moat common type of apparatus for the separation of the particulate from the gaseous phase incorporates a paper filter upon which the solid is deposited and through which the gases pass. Several of these devices perform well (4). Paper filters, however, are subjert to specific limitations ( 8 ) and meet only requirements A and B. The paper filter agglomerates the dust, has low temperature limits, and contaminates the particulate sample with organic material. The paper filter also produces a rather high pressure drop, which often seriously limits the rat.e at which the sample can be taken. This limitation often makes isokinetie ssmpling of stack gases difficult. Ifwater is alloared to condense on the filter paper, the gaseous phase is subjected to an undesirable scrubbing action. Condensation of water also causes very high pressure drops, which weaken the filter and increase the tendency to rupture. The determination of the amount of collected dust on the filter paper presents a problem that has not been solved to the satisfaction of this laboratory. The paper of the filter abmrbs some of the gaseous compounds and thus provides a false separation of the gaseous and solid states of a particular element, especially if very lam concentrations are encountered. The paper filter, however, does provide a most convenient and expeditious method of separating the gaseous and solid phases. Because of the limitations of the paper filter, this laboratory designed a sampler that operates on the electrostatic principle. This sampler was based on a combination of theory and practice. Equations and references are listed by Perry (S), hut i t was found that the final design . was best reached by the process of trial and error, because of the numerous variables imposed by the changing chemistry of the dust and gas encountered during stack gas
The gas enters through the tangentially positioned ‘/4-inch pipe at the lower end of the collecting tube. Any very large pieccs of solid material are separated by the slight centrifugal action itnd are found resting on the bottom plate a t the oompl? tion of the sample. The collecting tube is in tho farm of a truncated cone, in order to afford n more uniform distribution of dust in the bottom third to half of the tube. As the dust progresses upward through the
Fipire 1. 829
Stack sampling setup
ANALYTICAL CHEMISTRY
830 tube, i t is subjected to an electrostatic field of increasing intensity. Because.of the small diameter of the exit end of the sampler, the field intensity has reached the point of breakdown in normal atmosphere a t approximately 20 kv. Cylindrical tubes were found to provide very thick dust deposition in the lower fourth of the tube, resulting in excessive blowoff after approximately 1 gram was collected, and thus seriously reducing the sample volume for which high efficiency was afforded. Many samples of open hearth effluent have been taken with excellent efficiencies, when as much a~ 10 grams were collected in the truncated cone collecting tube. The negative discharge electrode consists of a O.02T-inch stain-
(5''I D
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Figure 2.
Collecting tube and plates
less steel wire, fixed to the top insulator (Figure 3) by means of the stainless steel bushing incorporating a side setscrew. The bottom insulator has a 0.032-inch blind hole drilled */*inch deep, which provides a support for the free end of the electrode. Electrically, Teflon is ideal for its application here, in that its dielectric strength in the presence of a corona is 50 to 75 volts per 0.001 inch. I t s electrical properties do not change over the wide temperature range of - 100' to +500° F., and its surface resistivity is high a t high humidities. It was because of these properties as well as its inertness to corrosive gases, that Teflon was chosen for the insulators; however, many ceramic materials will suffice when the gases sampled do not contain fluorides. An important consideration in the choice of an insulator material is its dielectric properties, to withstand the high potential. A single insulator unit was constructed and tests proved that the free end of the electrode was subject to excessive oscillation, resulting in arcing and low efficiencies. The lower insulator is provided with a small platinum-rhodium thermocouple entering from the lower end of the insulator. The thermocouple hot junction is exposed through the side of the insulator a t the point of entrance of the gases. The temperature of the inlet gases is thus determined by use of a conventional potentiometer. The incorporation of a thermocouple within this insulator is important because of the tendency of the Teflon (polytetrafluoroethylene) insulators to decompose. If fluorides are to be determined in the solids and gases collected, the temperature of the inlet gaaes must be kept below 350" F. However, if a slight amount of gaseous emissions from the insulators can be coped with, the minimum gas inlet temperature can be raked to 500" F. Other temperature properties of Teflon as listed by the Graef Engineering Co., Paramount, Calif., are its change to a transparent gel a t about 620' F. and its decomposition into gaseous constituents a t approximately 750" F. Trflon will not support combustion. An electric heating jacket was designed in order to maiiitain the desired temperature within the precipitator and is shown unmounted in Figure 1. If stack gases are being sampled, it is necessary to maintain temperatures ahove the dew point. S o r mally this temperature falls between 100" and 200" F. At these temperatures the life of the insulator should be indefinite. Open hearth effluent is considered to have many undesirshle electrostatic precipitating characteristics: however, several hundred samples of this effluent have been taken with this sampler and excellent efficiencies have resulted. To test the efficiency of dust separation of the sampler, a holder incorporating an 11.0-em. filter paper (Whatman No. 32 or 42) was placed on the exit of the recipitator. These papers are extensively used in flat filter horders for filtering dust-laden gas streams, because of their tight structure (8). In only a few cases was there any observable red color due to the iron oxide dust added to the paper, and in no case could the addition to the paper be detected on a laboratory analytical balance. The open hearth effluent upon which these efficiency studies were made consisted of materials collectable on Whatman S o . 32 or 43 papers. Approximately 85% of this material was under 5 microns in diameter. All electrostatic precipitators have the disadvantage of producing oxides of nitrogen and ozone. Perry (5)notes that ozone formation is less from a positive discharge electrode than from a negative discharge. Beadle, Kitto, and Blignaut ( I ) state that negative ionization produces 10 times as much ozone as positive ionization. HoFever, negative ionization occurs a t a lower applied potential and generally has a higher sparking potential and less erratic behavior, resulting in better collection efficiencies. It appears that positive ionization has only the advantage of lower ozone production, which should be considered when the presence of large amounts of ozone is not desirable. The effect of the generated oxidizing agents on the gas-solid mixture t o be sampled should be carefully evaluated before electrostatic methods are used. To date, no deleterious effects have been detected in open hearth stack sampling, although this oxidizing interference is probably present when a mixture of air and organic gases is sampled. The use of electrostatic methods involving mixtures of combustible substance3 and air should be avoided. LITERATURE CITED
(1) Beadle, D. G., Kitto, P. H.. and Blignaut, P. J., Arch. Ind. IIyg. and Occupattonal itfed., 10, 381-9 (1954). (2) Chambers, L. -4., A m . Ind Hyg. Bssoc. Qztart., 15, 290-6 (Decem-
BOTTOM INSUL&TOR - T H L R M O C W F t E NOT S H O W N
TOP
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Figure 3.
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IN
5 L A5SEMBLY
Insulators and bushing
q.
ber 1954). (3) Perry, J. H., "Chemical Engineers' Handbook," 3rd ed.. pp. 103945, AIcGraw-Hili, Sew York. 1950. (4) Stanford Research Institute, Stanford, Calif., private correvondence. R E C E I I E Dfor r e ~ i e wSeptember 20, 1954.
Accepted February 7. 1955.
