A
mechanical system was needed for removing radioactive particles of matter from the flue gases of an incinerator which would dispose of radioactive combustible waste. The system must work at heretofore unobtainable high efficiencies in the submicron range of particle size because of the extremely small quantities of radioactivity that can be permitted to escape into the atmosphere with the flue gas. Tests with various types of condensing steam nozzles indicated that efficiencies on the order of 99.9% could be obtained on 0.3-micron smoke particles. Steam nozzles, with other conventional scrubbers and the Chemical Warfare Service filter, have been used to process flue gas from a pilot
plant waste incinerator. Total penetration of the system by radioactive particles has been on the order of 0.000001%. The use of specially designed steam nozzles for removing small particles from a moving gas stream is something new in the field of particle removal equipment. The use of the double-orifice nozzle, which gives efficiencies on the order of 99.9 70 on 0.3-micron particles, is definitely limited to installations handling no more than a few hundred cubic feet per minute of gas. Standard steam exhausters, which give efficiencies on the order of 90% and better on 0.3-micron particles, may be used with very large gas flows, if steam is available at moderate cost.
P. J. Schauer MOUND LABORATORY, MONSANTO CHEMICAL CO., MIAMISBURG, OHIO
Removal of Submicron Aerosol Particles from Moving Gas Stream Condensation Center Effects in Steam Nozzles
M
AXUFACTURERS and the public alike have become increasingly conscious of the dangers from air pollution, not to mention the nuisance it has become in some localities. This problem is not confined to thickly populated areas, but also applies to many rural areas where crop damage may be caused. Many processes for eliminating the particulate matter from the effluent gas streams of manufact'uring processes have been investigated. During and immediately folloxing World War 11, the advent of large installations dealing vith radioactive chemicals and t,heir processes added new impetus and direction t o these investigations. One such investigation is herein described.
The Problem The problem of disposing of solid \Tastes from laboratories and plants handling radioactive chemicals can be lessened coneiderably by burning the combustible solids. The major difficulty is, of course, the cleaning of the flue gases from the incinerator, so that t,hey may be expelled to the atmosphere practically free of radioactivity. The flue gases from such a process may be assumed (in most cases) to contain this radioactivity in the form of particulate matter. The size of these particles varies over a wide range, and, in the absence of specific data, loading of submicron particles in the gas must be assumed to be high. The particles consist for the most part of carbon in the form of soot, mineral material (silica) in the form of fly ash, and hydrocarbon as smoke. The gas volume t o be handled is small, probably less than 500 cubic feet per minute. The use of oxygen-enriched air for burning, which is desirable in some cases, would further reduce the volunie of flue gas. The efficiency of removal required depends on the kind of radioactive elements involved. In this case efliciencies of practically 100yo are required.
Method Survey A brief survey of the literature shows that submicron particles have been removed from moving gas streams wit,h considerable success by means of electrostatic precipitation, filtration through porous barriers, impaction and diffusion of particles on the surface of liquids, and agglomeration by sound waves followed by centrifugal separation.
Various equipment and processes for accomplishing particle removal by these mechanisms are available commercially and have been described ( 4 ) . Each of these processes has many disadvantages when, applied to this incineration problem. Electrostatic precipitation would require many safety devices, even though dilution were used, because the gases might be explosive (carbon monoxide and excess oxygen). The cleaning and repairing of precipitators contaminated with radioactive particles would present a serious problem. Also, it was not known exactly what efficiencies could be expected from such a system. The Chemical Warfare Service filter (porous barrier type), although excellent on most submicron particle filtration from gas streams, had been shown in previous tests a t this laboratory to be only about 99% efficient on radioactive particles of submicron size. Placing a number of these filters in series does not sufficiently increase the efficiency. Venturi and Pease-Anthony scrubbers, acting on the impaction-diffusion principle, do not have high enough efficiencies. Because data on sound agglomeration were sparse and somewhat vague a t the time this investigation w a ~ initiated, and development costs were high, this method 11ad eliminated. Although a combination of some of the above processes might have given satisfactory efficiencies, a new method, the condensation center idea, showed promise of being vary efficient, simple, and inexpensive. It a a s therefore decided L O investigate this method first and, if it proved to be effective, t o combine it JTith some of the above-mentioned mechanisms to obtain the efficiencies required. The collection of the particles in water is advantageous because a process for liquid waste disposal is available.
Principle of Operation It is well known that water vapor will tend to condense much more easily upon reaching its dew point if ions or small dust particles are present to act as nuclei for the water drops formed. White (6) states that the prescnce of mch particles is, indeed, a necessary condition for condensation. Pellott ( 6 )thinks that coaggregated molecules of air probably act as nuclei for the drops formed in the condensation of steam in convergent-divergent nozzles. Probably the best known application of this phenomenon is the tracing
1532
INDUSTRIAL A N D ENGINEERING CHEMISTRY
July 1951,
of alpha- and beta-radiation paths in the Wilaon cloud chamber. Endebrock (1) thought that the same principle might be applied in a steam nozzle for a different purpose-Le., forming water drops around small particles in a gas stream, so that they may be easily removed or separated from the gas.
1533
mixing occurs. However, because of rate lag effects and heat losses, some further condensation is taking place throughout the length of the nozzle as the steam continues t o expand and cool. The effects of impaction and diffusion should be great where the streams of gas and steam first mix. The entering gas and its particles would be subjected to a terrific bombardment by multitudes of very small water droplets (in the Angstrom range) traveling a t speeds exceeding that of sound. This effect plus that of condensation center effects further along the barrel of the nozzle might possibly give high efficiencies with standard steam exhausters. Therefore, the first tests were carried out with this type of nozzle.
Use of Standard Steam Exhauster on Incinerator Flue Gases
STEAM INLET
’
Figure 1. Commercial Steam Exhauster
Steam, in expanding through a simple convergent-divergent nozzle, completes an adiabatic cycle. The energy used to obtain the high velocities involved in such nozzles is obtained at the expense of pressure and temperature losses and condensation takes place. If the expansion proceeds far enough, the velocity energy is recuperated, so to speak, the pressure and temperature Qf the gas rise, and the condensed particles reevaporate. The condensation in a nozzle of this type appears abruptly and very near the orifice. However, because of rate lag effects and heat losses, some Condensation occurs further along in the expansion. If, now, the nozzle is so built-Le., as a steam exhauster-so that another gas stream containing small particles of matter in suspension may enter and mix with the steam very near the or%ce, and if this gas stream is a t a considerably lower temperature than the steam stream, we may expect some condensation on the particles present. We may also expect this condensation to be stabilized to a certain extent by the heat losses occurring from the mixing of the hot steam and cool gas streams. If wet steam were used, further stabilization might be effected, because of the availability of the water drops in the wet steam to absorb some of the energy released by the velocity decrease toward the mouth of the nozzle. The water droplets emerging from the mouth of the nozzle could be made to grow large enough in a condenser system so that they could be removed almost completely by conventional equipment from the gas stream. Steam ispirators or exhausters, as made commercially, have their suction ports placed in such a position that the gas stream is mixed with steam a t some distance from the steam orifice (Figure 1). In all probability most of the steam has condensed before the
A small conventional firebrick incinerator was constructed for burning radioactive wastes. It was fired by two gas jets beneath the grates, which were used as oxygen supply lines once ignition was effected. Oxygen was also supplied over the bed for secondary combustion. The incinerator was airtight and would hold a charge of about 15 pounds of waste, which consisted of wood, g p e r , cloth, rubber, plastic tubing, etc. A standard Schutteoerting steam exhauster (Figure I), capable of moving 20 to 30 cubic feet per minute of air under 2 inches of water vacuum a t 90 pounds gage steam pressure, was used to furnish draft for the incinerator. The flue gas-steam mixture from the nozzle was exhausted into a water-cooled expansion chamber. The sudden drop in temperature and pressure in this chamber caused more moisture to condense on the small drops exhausted by the nozzle. Many of these drops then became large enough to fall out in this chamber and were drained off to a collecting tank.
Figure 2.
First Incinerator Pilot Plant
The noncondensable gases and the smaller entrained drops were then passed through a combination spray and packed column until the fog was eliminated and the noncondensable gases passed out into the atmosphere. The spray water from this column was collected in tanks. A known amount of the escaping gas was continuou?ly withdrawn from the stream through a standard air-
1534
INDUSTRIAL AND ENGINEERING CHEMISTRY
sampling apparatus for determining amounts of radioactivity in t,he gas. Figure 2 is a schematic picture of this process.
,4series of five runs made on this apparatus and an average efficiency of 98.80% was obtained. This represents the 0ver-d efficiency on particles of all sizes (size distribution and loading
Figure 3.
Nozzle-Testing Apparatus
unknown). Because all the known errors in this system would tend to lower the efficiency of the process, the results looked promking enough t o warrant further investigation.
Laboratory Tests and Nozzle Design Obviously, the next step was to find out exact'l~ where the condensation occurred in the simple convergent-divergent type of nozzle used in the incinerator tests. If most of the condensation had taken place before the flue gases entered (as was suspected), better results might be expected from a nozzle design in which the sequence of condensation and gas entry was reversed. To this end the apparatus shown in Figure 3 was assembled. This apparatus is very much like that used by Yellott in his studies of condensation in steam nozzles (6). The nozzle consisted of a brass channel, rectangular in cross section, 15 inches long, 1 inch wide, and 0.5 inch deep. Inserts of various configurations may be placed in this channel to form nozzles of various shapes (Figure 4). The removable top of the channel was recessed to take a 0.25-inch borosilicate glass plate and was cut away through the top to expose a glass window 9.25 inches long and SECTlON 'r' 'r'. 1 inch wide. This glass formed the top of the nozzle, the brass channel formed the bottom, and the inserts formed the sides of the nozzle. Air entry ports were cut into the side of the brass channel. The top piece, containine. the window. was held in d a c e on gaske& by screws to the channel eketion. Brass flanges were
kzle,t:22which Zptelisfeeasily ~ ~ disassembled.
g:
~ ~ h y type of nozzle may be tested by merely placing the particular shape of insert desired in the assembly. These inserts are simply and easily made (of brass) on an end mill. ~~~h is plated lvith a nickel-zinc alloy to give a dull black finish.
Vol. 43, No. 7
A beam of light from a zircon arc lamp can be focused through a glass window in the pipe tee a t any point within the nozzle or beyond t8henozzle exit. Thus :r strong pencil of light can be formed t'hroughout the lengt'h of the nozzle. Condensat'ion in the nozzle is observed through the glass plat'e window against the black background of the plated inserts and a t right angles to the beam of light,. X o light can be seen when live steam, air, or both are present, t'he nozzle appearing totally dark. However, when condensation occurs, light is immediately visible in the condensation region because of the reflection and/or refraction of the light bean1 by the small water droplets. The color and intensity of t,he light thus reflected are an indication of the size and amount of water droplets present. The steam pressure on the nozzle was controlled by nieans of a throttling valve in the 120-pdund steam line. The superheater consisted of a gas burner under steam coils and could be regulated to give any desired amount of superheat to the steam. TWh the gas burners off, wet steam x-as supplied t o the nozzle. The nozzle was exhausted to a large open chamber (two 30-gallon drums welded together) with a surrounding water jacket (two 50-gallon drums) for cooling. The heat exchanger was a tube-and-shell type with a double pass. The expansion chamber and heat exchanger were used to effect the grom-th of the small Tmter droplets emerging from the nozzle, so that they could be removed from the gas stream by the Pease-Anthony scrubber ( 2 , 3). Condensate from the expansion chamber and heat exchanger was removed DRAIN by a small pump. ,4ir flows were measured by a n anemometer. The nozzle may take its suction directly from the room or from t,he top of the superheater as a Source of higher temperature gas. Angle of divergencc of all nozzles tested was between 10" and 120. Orifice sizes used were o.26 and o.125 inch, Position of suction ports was varied from within 0.25 to 1.5 inches froin orifice. The distance from the orifice to the diffusion throat was varied from 1.875 to 2.625 inches. Steam pressures from 0 to 120 pounds gage were used. Steam was used in wet, saturated, and superheatbd conditions. The nozzles Tvere tested w7hile aspirating air (from 70" to 200" F.) and with their suction ports closed off,
--
SIDE VIEW OF STEAM NOZZLE.
Figure 4.
OASUET
Experimental Steam Nozzle
The results of the observations on convergent-divergent nozzles are shown schematically in Figure 5 , A. In everv case, no matter ~ ~ ~ ~ what the physical configuration of the nozzle or the condition of steam used, the condensation area was unaffected-i,e,, condensation, when it occurred, took place in the same area relative t o the orifice. As can be seen by Figure 5, A , this condensation occurs
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1951
1. Good suction but poor double condensation (large second orifice)
iA --=-
--
.. -
AIR ENTRY
Dioctyl Phthalate Smoke Machine
I B L
-_-
-
_-
J
---4
- -STEAM
--
-=ry-
A
AIR ENTRY
Figure 5.
1535
Condensation Areas
Shaded areas represent condensation, white areas represent live steam (transparency)
very soon after passing the orifice. The conditions under which condensation occurred varied with the different nozzles of this type, but the place of initial condensation, in relation t o the orifice, remained constant whenever condensation occurred. It was also apparent that most of the condensation in the nozzle occurred a t the initial condensing point and very little occurred further along the barrel toward the exit. The following conclusions were drawn from the tests: The steam exhauster used in the incinerator tests was accomplishing the removal of particles by an impaction-diffusion process and not by condensation center effects. Because the condensation in this type of steam aspirator occure so near the orifice, it is impossible to admit the noncondensable gases (through suction ports) and mix them with the steam before condensation occurs.
AND
The dioctyl phthalate (D.O.P.) smoke machine, which was developed by Edgewood Arsenal, was borrowed from the Chemical Corps, U.S.A., and assembled for nozzle tests. I n general, the machine manufactures smoke particles of constant size, measures their size optically, delivers the smoke to a filter at constant volume and concentration, and gives an accurate assay of smoke penetration of the filter to as low as 0.01%.
The smoke is manufactured by heating liquid dioctyl phthalate and sweeping the vapors into a cooler air stream, thus quenching the vapors and forming smoke. The smoke is diluted with a constant volume of room air, and delivered to the filter being tested. A sample of this smoke stream is continually withdrawn through a n optical instrument called an "owl," which gives a direct measurement (visually) of particle size. The smoke penetration of the filter is determined by the use of a light-scattering chamber, a photoelectric cell, and a Naval Research Laboratory smoke penetration meter.
Particle Removal Apparatus The equipment-used for the nozzle condensation tests (Figure 3) was used for the particle removal tests. In order t o test this system with the smoke machine previously described, it was necessary, in effect, to substitute this system for the small air type of filter usually tested in the machine.
Observations of a double convergent-divergent type nozzle were made next. It was hoped that by using a double orifice in The suction ports of the nozzle were connected directly (in case of simple convergent-divergent type) or through two pumps (in which the second orifice was larger than the first the water dropcase of double convergent-divergent type) to a hollow wooden 1ets.formed at the first orifice might be made t o evaporate upon box placed in the smoke machine where a filter would ordinarily approaching the second constriction, so that the noncondensable be placed. I n this way the nozzle can take any amount of smoke gases (admitted a t first orifice) might be in contact with live steam required as the smoke stream from the machine passes through in the area of the nozzle preceding the second constriction. Condensation after the second orifice might then accomplish what the initial condensation failed t o do-i:e., form water drops about any particles present as nuclei. Figure 5, B , shows schematically the effect obtained in this type of nozzle. The condensation areas mere obtained as desired, but in so doing an inherent disadvantage of this type nozzle was encountered. The simple NOZZLE NO.1 convergent-divergent nozzle acts as an aspirator, moving the noncondensable gases through the nozzle. However, these suction characteristics are immediately impaired by adding a second orifice. As the crosssectional area of the second orifice approaches that of the first (diminishing and within limits), the evaporation and second condensation effects become increasingly better while the suction characteristics become increasingly worse. Three nozzles of NOZZLE N0.3 NOZZLE N0.4 this type were set aside for future particle removal tests. The performance of these nozzles was: Figure 6. Nozzle Types Tested with Dioctyl Phthalate Smoke
INDUSTRIAL AND ENGINEERING CHEMISTRY
1536 Table I.
Sozzle 1 2 3 4 5 1
90
Steam Condition Wet Wet Wet Wet We:
Flow Cu. Feet/ Min.
15 10 15
13
30
Particle Concn. Particlea/ Cu. Foot
Particle Size, Micron
5.9 5.9 5.9 5.9 5.8
0.3 0.3 0.3 0.3 0.3 0.3
13
15
(Superheat)
WATER SUPPLV
Limitations of Testing Process
Penetration Efficiencies of Various Types of Nozzles Smoke
Steam Pressure, Lb. Gage 60 60 60 40 90
X X X X
Vol. 43, No. 7
The dioctyl phthalate smoke machine has two serious limita0.03 8.0 8.0 tions in so far as these nozzle 0.06 4.0 4.0 0.04 3.0 3.0 tests are concerned. I t would 0.20 0.35 0.15 be of great interest to know 0.20 7.0 6.8 0.80 30.0 29.2 the efficiency of the particle removal system on various size ranges of particles. Unfortunately, the-dioctyl phthalate machine will produce particles only within the narrow range of 0.15 to 0.40 micron. These tests were therefore carried out on one particle size only (0.3 micron). The effects of particle loading in the gas stream would also be of great interest. Ho-xever, the smoke machine was so constructed that it. was considered impractical t o modify it t o the extent of being able to vary the smoke concentration and measure it accurately. Therefore, the same concentration is used in all tests. The particle loading given in the data is a computed value from the average amounts of dioctyl phthalate consumed and the particle size. It should be considered correct only in so far as order of magnitude is concerned,
Assay, % Blank Total
10" 1011 10"
10"
X 10"
5 . 9 X 1011
BY-PASS VALVE
CWLING WATER DISCHAROE
Penetration,
%
Test Results
Figure 7.
Figure 6 shows simple sketches of the configurations of the various nozzles tested, and Table I gives the comparative effectiveness of the nozzles on particle removal. The results shown are the best obtainable with each nozzle. Each nozzle was tested using steam pressures from 0 t o 100 pounds gage and both wet and superheated steam. The results shown in Table I were obtained in each case from a t least 1 hour of continual operation with a maximum fluctuation of = t O . l % ,
Gas Process Equipment of Second Pilot Plant
A. Rough washer
B. Heat exchanger (water-cooled)
Nozzle 5 is a commercial steam exhaust'er of the same type a s nozzle 1, and is the nozzle used in the original incinerator pilot pla,nt runs. Two tests were carried out with nozzle 1. In one test the conditions mere such that condensation took place in the nozzle. However, it had been not'ed that no condensation occurred in t,his nozzle if the steam were superheated about.30"; all condensation occurred in the expansion chamber. A probability existed then that the particles acted as nuclei for condensation in the expansion chamber instead of the nozzle. As can be seen from the table, the results were very poor. The results of these tests show conclusively the value of double condensation effects. Sozzle 1 has good suction characteristics and no double condensation. The mechanism involved here is probably one of impaction, diffusion, or bot,h. Nozzle 2 has a slight double condensation, and its particle removal efficiency is slightly better. I n nozzle 3 the suction characteristics are beginning to be impaired by the condition which causes double condensation. Results here are improved over nozzle 2 . Nozzle 4 has no suction but very good double condensation and, as results show, is excellent for particle removal.
H. Heat exchanger (water-cooled)
1. Pease-Anthony type scrubber J. D. Gas pum (Nash) K. E. Heat excganger (water-cooled) P. F. Steam nozzle T G. Expansion chamber (water-cooled)
C.
Pease-Anthony type scrubber Blower CWS filter Pressure gages Thermometers
the box. The excess smoke from the machine was piped to a n unused hood, where i t was dissipated into the ventilating system. The effluent smoke assay line to the scattering chamber was relocated a t the exit of the Pease-Anthony scrubber. After the above installation was made, the f i s t test to be run was a blank of the apparatus. If all the entrained water drops were not being removed by the Pease-Anthony scrubber, the PU'RL meter would show a reading, because some of these small drops would be drawn through the scattering chamber. With a nozzle aspirating filtered room air, the variables of the apparatus were adjusted (cooling water, steam temperature and pressure, and air flow) until the lowest blank of 0.04% was obtained. Stray light in the scattering chamber could not be entirely eliminated, so that a blank of 0.02% was obtained there. Hence, the total blank of both smoke machine and removal apparatus was on the order of 0.06%, n.hich was considered permissible. Because this blank a-as affected by some of the operating variables, it was necessary to use a blank on each nozzle tested by this apparatus.
Table 11. Pressure drop, inches HxO Spray water, gal./hour Cooling water, gal./hour Seal water, gal./hour Steam consumption, lb./hour Gas flow, OLI. feet/min. a
Series flow.
Rough Washer Negligible 50
... ...
... ...
1st Heat Exchanger Xegligible
Scrubber 6 40
... ...
..
...
Equipment Operating Data
1st P-A
.. ..
.4pp*ox. 1000'
..
Process Application to Incineration On the basis of the nozzle test with dioctyl phthalate smoke and the previous incinerator tests, another pilot plant for disposing of contaminated wastes was constructed. The design, construction, and operation of the incinerator are not discussed here, as they are considered beyond the scope of this paper. The process-
Piash
Pump
... ... i io ... ...
Steam
Expansion Chamber Kegligible
2nd Heat Exchanger Negligible
...
Agprox. 1000
A'pprox. l O O O a
Nozzle 27 lb. gage 5
2501300
...
... ... ...
... ... ...
2nd P-.A ScrubbcE 6 20
..
1.
CWY Filter
1
...
... ...
2i,' standard conditions
fuly 1951
R u n No.
'
Waste Burned,
Lb. 15 25 35 45 45 30 25
Time of Burning, Min.
Inctineratora,
%
40 60
1537
Table 111. Activity Removal Efficiencies Nozzle System Rough P-A Nashb Washer,
% 68.7 51.4 78.6 62.0 78.8 77.7 50.1
Scrubber,
% 96.7 91.3 83.6 82.6 79.4 73.4 57.0
Compressor,
% 89.0 79.3 87.0 89.7 84.4 81 6 66.0
75 75 75 60 75 75 75 90 75 75 75 75 Tests indicated t h a t approximately 75% of original activity i n waste remains in ash in incinerator. 7 9 10 11 12 13 14
a b
INDUSTRIAL AND ENGINEERING CHEMISTRY
CWS Filter,
(Stea 11 PI, %
% 99.9 99.9+ 99.9+ 99.9+ 99.9+ 99.9+ 99.9+
78.6 (48lb.) 92.5 (52lb.) 97.0 (56lb.) 99.2 6 O l b ) 99.8 I60 Ib:)
Over-all Decontamination Factor 107 t o 108
107 10' to 108 108 108 t o 100 108 108
Activity picked u p in aeal water of compressor.
(Pease-Anthony type) which follows tho heat exchanger. Agas sample is taken a t this point to be assayed for radioactivity, the gases then are assed through a blower and a Chemical g a r f a r e Service f i l t ~ r ,a final assay is taken, and the cleaned gases are led to the stack.
AIR ENTRY
Pilot Plant Tests with Dioctyl Phthalate Smoke The pilot plant described was tested with the dioctyl phthalate smoke machine Figure 8 . Steam Nozzle of Second Pilot Plant previouslv described. It was found that. Dimensions of nozzle with the "Chemical Warfare Service filtei 1st orifice 0.283 inch diameter Exhaust l l / n inches diameter removed from the system, a removal effiOver-all length 1 5 5 / 8 inches 2nd orifice 0.400 inch diameter ~ be obtained using ciency of 9 9 . 9 0 ~cquld Angles of divergence 1l0 Water spray openin 8 (4) 0.0135 inch diameter Air entry openings (2) 8/s inch diameter Angles of convergence 1Io a minimum steam pressure of 48 pounds Steam entry 1 inch diameter gage and a gas flow of 25 cubic feet per minute. When the steam flow to the nozing of the contaminated flue gases from this incinerator is briefly zle was cut off, the removal efficiency immediately dropped t o as follows (Figure 7): zero, thus showing the ineffectiveness of scrubbers of conventied design in removing 0.3-micron dioctyl phthalate smoke The motivating power which forces the flue gases through the various pieces of equipment is a Nash water-sealed compressor driven by a 10-hp. motor. The suction side of the compressor i8 Pilot Plant Tests with Radioactive Materials connected to the flue of the incinerator through three pieces of Seven tests of the Pilot plant have been made, I n each test, equipment: a rough washer a heat exchanger, and a scrubber of the Pease-Anthony type. fi'he rough washer subjects the hot radioactive waste consisting of paper, cardboard, rubber gloves, flue gases to a coun~ercurrentwater spray immedlately after rubber and plastic tubing, cloth, Lucite and vinylite plastics, and these gases leave the incinerator. This s ray not only washes the wood was burned. The waste was charged into the incinerator in gases free of large particles of ash ,,f soot but cools the separate packages weighing from 2 to 4 pounds .each. Air, gases considerably. The gases next pass through the heat exchanger, where they are cooled to room temperature before entering the scrubber. This scrubber removes practically all remaining particles larger than 2 t o 5 microns, so t h a t the gases entering the pump are cool and free of large particles. The compressor now forces the gases through a n air-cooled heat exchanger and thence into the steam nozzle. A proportional sample of this gas is removed prior t o entering the nozzle and assayed for radioactivity. This is the first gas sample assay taken, because the large particles present in the gas stream before this point would cause the assay to be in error. The steam nozzle used in this pilot plant is shown schematically in Figure 8 and by photograph in Figure 9. The contaminated gases are pumped into the nozzle through the four gas ports and are mixed with the partially condensed steam beyond the first orifice. As the mixture of steam, water, apd gas approaches the second orifice, the condensed portion of the steam evaporates, so t h a t the contaminated gases and live steam are in intimate contact before passing the second orifice. Immediately upon passing the second orifice the steam condenses, utilizing Figure 9. Steam Nozzle of Second Pilot Plant any small particles present as nuclei for condensation. Four very small water jets are then injected into this stream to help stalillize the condensation, and the entire mixture is exhausted to a large waterslightly enriched with oxygen (33% total oxygen), was furnished cooled tank (expansion chamber). The mouth of the nozthe incinerator for combustion. Temperatures of the flue gas d e is placed 8s near the expansion chamber as possible, SO that immediately after leaving the combustion area ranged from 1 0 0 0 ~ there is no appreciable velocity decrease until the chamber is to 18000 F. Equipment operating data are given in Table 11. ~~~~a~~~ Individual efficiencies (radioactivity removal) of the various changer immediately following i t causes more moisture t o be conPieces of equipment are given in Table 111. The over-all effidensed on the small drops already formed, SO t h a t these drops beciency of the process is given as a "decontamination factor," a come large enough to be removed by the conventional scrubber term used in lieu of a percentage figure where the percentage is
~
~
~
"
,
e
~
~
~
~
~
I
~
1538
INDUSTRIAL AND ENGINEERING CHEMISTRY
extremely high. A decontamination factor of 104 would be equivalent to B removal efficiency of 99.99%, 106 to 99.999%, etc. The amounts of radioactivity involved in these tests varied widely, but the minute amounts escaping the system remained practically constant in each test.
Vol. 43, No. 7
used in the first pilot plant. This would tend t o support t h e theory t h a t a steam exhauster of that type acts on an impactiondiffusion principle, as do these scrubbers.
Acknowledgment
Conclusions The efficiency of the nozzle system in removing radioactive particles from the flue gas stream has steadily increased. The increase from 78.6 to 97.0% can be attributed to the higher steam pressures. The increase from 97.0 t o 99.Syo was accomplished by operating the nozzle without its water jets and by improving the action of the second Pease-Anthony scrubber. The efficiencies of the Chemical Warfare Service filter and of the Nash compressor (for particle removal) have been much better than expected. On the basis of these pilot plant results, a full scale incineration process is n o v being designed. The over-all efficiency of the rough washer, Pease-Anthony scrubber, and Nash compressor (combined) is of the same order of magnitude as t h a t obtained from the standard commercial-type steam exhauster
The author wishes to express his thanks to Fred M. Huddleston, Paul ?*I.Hamilton, and Frank C. Mead for their help in making this paper possible.
Literature Cited (1) Endebrock, R. W., Mound Laboratory, Miamisburg, Ohio, un-
published report.
(2) Kleinschmidt, Chem. h M e t . Eng., 46, 487 (1939). (3) Kleinschmidt and Anthony, Trans. Am. SOC.Mech. Engrs., 63,
349 (1941). “Chemical Engineer’s Handbook,” 3rd ed., p. 1031, New York, McGraw-Hill Book Co., 1950. ( 5 ) White, H. E., “Classical and Modern Physics,” p. 465, New York, D. Van Sostrand Co., 1940. (6) Yellott, J. I., Trans. Am. Soc. Mech. Engrs., 56, 417 (1934) (4) Perry, J. H.,
RECEIVED November 24, 1950. Work done under the auspices of the Atomic Energy Commission
Contract No. AT-33-1-GES-53.
Treatment of Radioactive Water by Phosphate Precipitation T h e work was done to check the effectiveness of a calcium phosphate floc in removing radioisotopes from large quantities of water. In almost every case the phosphate was found to be more efficient than either alum or ferric hydroxide. Maximum removals were obtained under conditions of high pH and i n the presence of an excess of phosphate. In general, good removals were obtained for those isotopes which would be precipitated
under the same conditions if they were present in macro quantities. While the data must b e considered preliminary in nature, they indicate that coagulation and filtration techniques, if performed under the proper conditions, can be applied to treat liquid wastes containing low levels of radioactivity. The total reduction obtained in the activity of the waste will b e dependent on the radioisotopes present.
R. A. Lauderdale OAK RIDGE NATIONAL LABORATORY, OAK RIDGE, TENN.
I
N T H E treatment of large quantities of water containing rela-
with a hydroxide, the floc can be formed in a solution of high
tively low levels of radioactivity, coagulation of hydrous precipitates offers a number of advantages. It is the normal method of treating municipal water supplies and could, therefore, be used in existing equipment in the event of an emergency. I n some instances, adsorption is one of the most efficient methods of removing low concentrations of impurities from solution. Thirdly, minimal, rather than massive, doses of reagents are used-an important factor economically. I n normal practice, thGh floes usually employed are aluminum hydroxide or ferric hydroxide. I n the treatment of radioactive water, however, it was felt t h a t other methods should be tested for use either alone or in conjunction with the normal practice. This report describes work that has been done using calcium phosphate precipitation as a method for treating radioactive wastes. The decision to investigate the use of a calcium phosphate floc was based on a comparison of the insolubility of the phosphates and hydroxides, which showed t h a t the number of highly insoluble phosphate compounds exceeded the number of insoluble hydroxides. This was considered desirable from the standpoint of Hahn’s adsorption and coprecipitation rule, which states that the adsorption and/or coprecipitation of a n ion by a precipitant depends on the surface charge of the adsorbing precipitate, and on the degree of insolubility of the adsorbed compounds in the solvent involved. I n addition to a n expected increased coprecipitation of radioactive ions by a phosphate floc, a s compared
pH, a condition favoring the adsorption of certain ions on the turbidity usually present in natural waters and the formation of radiocolloids. (Unpublished data of the author indicate t h a t natural clays are highly efficient in the adsorption of cesium ions, the efficiency increasing with the p H of the solution.) Furthermore, the phosphate floc has a very large surface area (as do the hydroxides), which favors the surface adsorption of ions and charged particles. The isotopes of immediate interest were uranium fission products and particularly those of fairly long half-life. For the initial tests isotopes were selected which would be representative of the different groups of the periodic chart. It should then be possible to extrapolate the data for a given element to other elements having similar characteristics. Tracer-type runs were made with the isotopes of cerium, strontium, zinc, yttrium, antimony, and tungsten. The phosphate floc was formed by adding the radioisotope and a solution of either potassium dihydrogen phosphate or sodium phosphate to a solution of calcium hydroxide in distilled water. The mixture of phosphate, calcium, and radioisotope was given a flash mix for 5 minutes, followed by slow stirring t o allow the newly formed floc t o grow t o a size which would settle. Following the slow mix, the floc was allowed to settle for 2 hours. The p H of the liquid was adjusted for some tests by the addition of dilute sodium hydroxide. Efficiencies of removal were calculated from
