I Air Pollufion
H. F. JOHNSTONE and A. J. MOLL1 University of Illinois, Urbana, I l l .
...
Formation of Sulfuric Acid in Fogs When nuclei of oxidation catalysts are present, sulfur dioxide fogs produce sulfuric acid. Action of manganese and iron salts was studied in simulated stack gases
THE
u
The box, 18 X 18 X 24 inches, \vas made of 0.5-inch Lucite plates sealed in the same way. Eight-inch holes were cut in the end plates of the cylinder and in the top and front plates of the box for ready access to the fog chambers. These were covered with 0.5-inch Lucite plates fastened with gaskets and eight 10-24 machine screws with wing nuts. Connections to the chambers were made with borosilicate glass 25/40 ground glass joints sealed in the hand-hole covers with Duco cement. The chambers held pressure o r vacuum u p to 10 cm. of mercury. The walls of the box were cooled by evaporation of water from moist cloths and by a copper cooling coil under the bottom. T h e cylindrical fog chamber had the advantages of easy control of conditions before the fogs were formed, nearly piston-like flow of gas while sampling the fogs, and slower fallout of fog droplets. In both chambers nuclei and fogs were observed by Tyndall beams from intense lights. The relative air humidity was adjusted to the desired value before the nuclei were introduced by passing a stream of saturated filtered air through the fog chamber from a humidifier as shown below consisting of a column of distilled water over a 0.5-inch Filtros plate in a 4inch Lucite cylinder with an internal electrical heating coil. Nuclei Generator. The nuclei were generated from solutions of the salts with
ABSORPTION and oxidation of sulfur dioxide from air by solutions of catalysts is well known. Wyld mentions early patents on the manufacture of sulfuric acid by catalysis of the reaction in solutions of copper, manganese, iron, and tin salts (75). The reaction has been considered for use in the recovery of sulfur dioxide from dilute waste gases such as sulfuric acid (5, 9). Andersen and Johnstone found that fritted bubblers are more efficient for absorption of the gas from air by 0.01M MnSOr solutions than venturi scrubbers ( 3 ) . Johnstone and Coughanowr ( 7 0 ) studied the kinetics of the absorption-oxidation reaction in drops of dilute manganese and ferrous sulfate solutions. From their data they estimated that the rate of conversion of 1 p.p.m. of SO2 to sulfuric acid in a fog nucleated by 1-micron MnS04 particles may be as fast as 1% per minute. When catalyzed by manganese sulfate, the reaction has an induction period and is inhibited by phenols and copper ions to such an extent that the reactants are not absorbed from bubbles when these inhibitors are present (9). The inhibitors are less effective when the absorption takes place at drop surfaces (10). In view of the effects of breathing air containing low concentrations of sulfuric acid aerosol (2, 4, 72, 7 3 ) and the possibility of synergistic action with low concentrations of sulfur dioxide ( 7 , 2, 8 ) . the reaction is important in air pollution. In the work described here, the formation of sulfuric acid from dilute sulfur dioxide in the presence of artificial fogs nucleated by small particles of manganese and iron salts was studied. These substances are often present in air as small particles of ash from the combustion of coal and fuel oil. The catalytic activity of suspensions of an ash in water can be determined easily by the method used by Johnstone (9). The effects of the presence of inert salt nuclei and of inhibitors in air along with the catalytic nuclei on the rate of reaction and formation of the acid
when relative humidity of air is below saturation, but above the critical value a t which saturated solutions of the salts exist were also noted. Laboratory experiments with artificial fogs formed in controlled atmospheres show that oxidation of sulfur dioxide in air a t concentrations COSresponding to those in combustion gases diluted 10 to 1000 times takes place rapidly when catalytic nuclei are present. Concentrations of acid up to 50 mg. per cubic meter are formed in a few minutes. High concentrations of inert nuclei decrease rate of formation, but do not stop the reaction. The fastest rate is with catalytic aerosol at relative humidities just below 100%.
Present address, Department of Chemical Engineering, University of Washington, Seattle, Wash.
Left. Humidifier column and box chamber Right. Cylindrical fog chamber with Tyndoll beam in light through lower hand-hole plate produced by acid fog in air below 100% relative humidity
Experimental Apparatus. The two Lucite fog chambers shown below were used. The volume of each was approximately 4.2 cubic feet. The cylindrical chamber was 10 inches in outside diameter and 104 inches high. It was jacketed with a 12-inch Lucite cylinder. Cool or warm air was passed through the concentric shell to keep the wall of the inner cylinder either at a constant uniform temperature or with a temperature gradient from bottom to top. The ends of the two cylinders were closed with 0.5-inch Lucite plates with grooves cut to fit the cylinders and sealed with an ethylene dichloride solution of Lucite.
Apparatus used for fog studies
VOL. 52, NO. 10
OCTOBER 1960
861
I"
FILTERED
the U before the reservoir. Air was drawn at a high velocity past the junctions. The potential differences between thc wet and dry junctions and between the dry junction and a reference junction in ice were measured on a precision potentiometer or on a continuous recorder. I n this way the relative humidity at any point in the upper half of the column could be measured quickly with an accuracy of 2 to 570. Sampling and Analysis of Fogs. The fog droplets were collected after fixed periods of time following the introduction of the sulfur dioxide, or after fog was formed by adiabatic expansion of thc air in the chamber. Two methods of sampling fogs were used. In some runs, a high velocity impactor similar to the one described by Ranz and Wong (14) was used. The dimensions of the jet orifice were 0.007 X 0.5 inch. The velocity of the gas through the orifice wras near sonic in all cases. The droplets were collected in a small glass cup held in position under the orifice, The impactor separated all of the droplets larger than 0.3 micron. A drop of phenol solution was placed in the cup to prevent oxidation of sulfur dioxide dissolved in fog water. In the other method a small glass cyclone was used. In both cases the amounts of sulfuric acid and sulfurous acid in the samples were determined immediately by titration with 0.02N h-aOH. Two methods were also used for titration. In one, methyl red with a small amount of methylene blue was the indicator; after the end point was reached, when the amount of sodium hydroxide neutralized was equal to twice the moles of sulfuric acid plus one equivalent of the sulfurous acid present, an excess of hydrogen pzroxide was added and the titration was continued to the second end point to find the number of equivalents of sulfurous acid. In the second method, phenolphthalein was the indicator for the first titration, afrer which the solution was acidified and the sulfurous acid was determined by titration with 0.01Y iodine solution. The observed composition of the fogs \\'as in-
1
AI,R
'I4
DETAILS
SWML FILTER FLASK
/= ;Jkj __ -
-
_ _ -
--
___
-
-
The nuclei were obtained from solutions of various salts in this generator
a simple modification of the generator described by Klumb ( I T ) sholvn in the figure above. The action of the generator appeared to be that of a submerged siren formed by air passing through the gap between the two tubes. The flow of filtered air to the generator usually was about 7 liters per minute. The siream leaving the generator passed through a glass trap and a 3j8-inch plastic tub: into the top of one of the fog chambers. The size of the nuclei could be varied by changing the concentration of the salt solution. The size distribution is shown on the graph. The size is in the range reported for condensation nuclei in natural clouds and fogs (7). Humidity Determinations. For the experiments in the box, the relative humidity of the air was measured by an electric hygrometer (American Instrument Co.). In the cylinder a pair of wet-and-dry thermocouples made of h-0. 30 B and S gage copper-constantan wires was used. The wet junction was placed in the long arm of a glass C-tube which extended through the top handhole plate. A thread wick extended from the junction to a water reservoir in the bottom of the U, which could be replenished from the top. The dry junction was placed in the short arm of
d:,>. = 0.722 microns
d,. = 0.795 microns = 0.783 microns d,.s = 0.81 9 microns a, = 1.170
d,
8 0
I
3
;I
CI
1 05
I
2
5
1
'
1 1
I l l
IO 23 32 40 M 60 70 80 90 9 5 98 P E R C E N T LESS T H A N D I A M E l ER SHOWN
5G995
I
9Y9
The size distribution of manganese sulfate nuclei was obtained b y observing the rate of fall in a Millikan cell of droplets formed b y 200 particles in an atmosphere of known humidity, and b y sizing 30 dry particles under a microscope at 1500 X
862
INDUSTRIAL AND ENGINEERING CHEMISTRY
dependent of the method of sampling and titration used. The amount of sulfurous acid was always negligible compared with the amount of sulfuric acid. Procedure. The fogs were formed by expansion of moist air in the prwcnce or sulfur dioxide and nuclei in onc of the chambers. After the air in the chamber was adjusted to the desired humidity, nuclei were added by operating ihe generator at a fixed rate for a fixed period of time between 10 and 60 seconds, so as to have comparable quantiries of the particles present. The amount of sulfur dioxide required to give the desired concentration was then introduced from a mercury-filled gas buret. The capillary line from the buret to the chamber was flushed with a small amount of filtered air which also served to mix the gases. The Tyndall beam showed that the nuclei were uniformly mixed in about 1 minute. Expansion of air in the chamber was accomplished by increasing the pressure by several millimeters of mercury above atmospheric pressure and removing the cap from one of the ground glass joints in a hand-hole cover. The amount of fog formed could be regulated by the initial relative humidity and the change of pressure. The expansion caused fog to form immediately and the time of reaction was measured from that instant. Residual currents from the expansion continued to mix the gases for a fe\v minutes. There was gradual subsidence of the fogs shown by the fall of an upper line of demarcation. From the rate of fall, the approximate size of the droplets was estimated to be between 5 and 20 microns (6). Fogs formed in the box usually disappeared within 15 minutes. When the walls were warmer than the supercooled gases, evaporation caused more rapid disappearance of water fogs because of transfer of heat from the walls. When the temperature of the walls was near that of the expanded gases, the thermal stability of the fogs was greater. SVhen sulfur dioxide and catalytic nuclei were present in the box at the time fog was formed, there was a noticeable change in the appearance of the Tyndall beam after a few minutes. This was probably caused by the partial evaporation of the fog leaving smaller droplets of sulfuric acid that remained in equilibrium with the partial pressure of water vapor in the box as the relative humidity decreased with temperature rise. These observations are important in predicting the behavior of natural €ogs containing sulfuric acid. A series of runs was made in the cylinder in which cooling by adiabatic expansion Tvas omitted and the droplets from the nuclei generator remained in suspension as small drops of concentrated solution in equilibrium with air containing moisture and sulfur dioxide.
AIR POLLUTION Table I.
Concentration of Sulfuric Acid in Fogs Formed by Expansion in the Presence of Sulfur Dioxide and Various Nuclei
Nuclei MnSO4
27.5 26.5 24.5 26.5
NaCl None NaCl
+ MnSO+
Mn(N0d~ MnCL MnSO, FeSO4
MnSOP
a
Conditions Before Expansion E ~ ~i~~ ~ After ~ ~ - &SO4 Rei. so2 sion, Expansion Time of in Fog, Temp., Humidity, Content, A p , mm. Started, Sampling, Mg./Cubic O c. yo P.P.M. Hg Min. Min. Meter
28.0
26.5 26.0 26.5 24.5 26.0 25.5 28.0 31.0 28.5 30.0
89 92 91 92 94 93 95 94 94 99.5 97 98 97 97 98.5
217 217 267 267 267 217 217 217 267 267 267 100 267 267 267 267
8 5 5 5
20 2 3.5 3.5 3 3 3 3 3 3 3 3 5 3 3 3
5 5 5 5 5 5 5 5
5 5 5
5
7 2 3.5
5 5 5 5
5 5 5
5 5 8 5 5 5
6.27 0.49 1.26 1.35 0 0 0.20 0.05 1.00 0.54 0.48 0.11 0.30 0.08 0.06 0.15
Approximately 10 p.p.m. of phenol vapor present in air.
2 2 2 2 2 2 2 2
250 250 250 250 250 4 50 500
86 95 95 95 94 97 96 97
2 2 2
0.54 7.09 20.5 38.7 49.5 0.5 8.6 41.1
FeSO4
2
250
96
2.5
10.7
N aC1
0.8 0.8
250 250
95 95
2.5 2.5
I n these experiments the relative humidity was between 80 and 98%. Reaction time was measured from the instant sulfur dioxide was first introduced into the chamber containing nuclei. The Tyndall beam showed that the aerosol consisted of particles larger than dry nuclei, but smaller than fog droplets formed by expansion. The effects of relative humidity, concentration of sulfur dioxide, and reaction time on amount of acid formed were found from these experiments. Several runs were made in the box to determine the effect of phenol vapor on rate of sulfuric acid formation in fogs. A small amount of a standard phenol solution was placed on a watch glass suspended below one of the ground glass joints in the top hand-hole plate and allowed to come to equilibrium with air before fog was formed. T h e odor of phenol was present in all cases. Discussion of Results The concentration of sulfuric acid formed in fogs under various conditions
1 3
5
Acknowledgment The authors thank James Gieseke, Charles Wolven, and Thomas Tung for the experimental work. literature Cited
Table II. Concentration of Sulfuric Acid Aerosols Formed from Sulfur Dioxide in Air in the Presence of Nuclei at Relative Humidities Below 100% Nuclei __ Gas Composition ~i~~ &SO4 Relative Before SO2 in Aerosol, SO2 Concn., 3Ip./Cubic Concn., Humidity, Sampling, Mg./Cubic Oxidized, Type Meter % Min. P .P.M . Meter % MnSOa 2 250 77 2 0.15 0.02 2 2
tion was less than it was when only manganese sulfate nuclei were present. Ferrous sulfate nuclei caused acid to form at a slower rate than nuclei of manganese salts. T h e presence of phenol vapor decreased the rate of acid formation, but did not stop catalytic reaction in the presence of manganese sulfate nuclei. This agrees with the results obtained previously with large drops (70). A small but definite amount of acid formed when sodium chloride nuclei only were present.
0.85 0.40
0.06 0.74 2.1 4.0 5.1 2.9 2.9 2.1 1.2 0.10 0.06
is shown in Tables I and I1 for the two sets of experiments. The initial concentrations of sulfur dioxide used are higher than those ordinarily found in air near the ground. Rather, they correspond to the concentrations in gases from the combustion of sulfur-bearing fuels after diluting 10 to 1000 times by mixing with air. The times of exposure of gas to nuclei and fogs likewise correspond to the persistence of plumes from stacks discharging gases into winds of 2 to 10 miles per hour, that is, to a plume travel of 0.25 to several miles. The reaction takes place in the liquid phase in the presence of catalysts. When there was no Tyndall beam showing the absence of particulate nuclei and when sodium chloride particles were the only nuclei present, no acid was formed in the expansion fogs. When manganese sulfate or ferrous sulfate nuclei were present, significant concentrations of sulfuric acid were formed in fogs within a few minutes. When both sodium chloride and manganese sulfate nuclei were present in about equal concentrations, the rate of acid forma-
(1) Amdur, M. O., Intern. J . Air Pollution 1. 170 (1959). (2) ’AmdGr. M’. O., Silverman, L., Drinker. P., Arch. Ind. Hyg. Occupataonal Med. 6, 305 (1952). (3) Andersen, L. B., Johnstone, H. F., A.I.Ch.E. Journal 1, 135 (1955). (4) Cullumbine, H., Pattle, R. E., Burgess. F., “Toxicity of Fog,” 7th Intern. Congr. Comparative Pathology, 1955. (5) CoDson. R. L.. Pavne. J. W.. IND. ENCL’CHEM. 25, 909 (i933). (6) Dalla-Valle, J. M., cLMicromeritics,’’ 2nd ed., p. 73, Pitman, New York, 1948. (7).Green, H. L., Lane, W. R., “Particulate Clouds, Dusts, Smokes and Mists,” pp. 370-7, Van Nostrand, Princeton, N. J., 1957. (8) Grohse, E. S., Saline, L. E., J . Air Pollution Control Assoc. 8 , 255 (1958). (9) Johnstone, H. F., IND. ENG. CHEM. 23, 559 (1931). (10) Johnstone, H. F., Coughanowr, D. R., Ibid., 50, 1169 (1958). (1 1) Klumb, H., Schwebstofftechnik Arbeitstagung, I Physikalische Instituts, Johannes Gutenberg Universitat, Mainz, West Germanv (1954). (12) Pattle, R. E., Burgdss, F., Cullumbine, H., J . Pathol. Bacteriol. 72, 217 (1956). (13) Pattle, R. E., Cullumbine, H., Brit. Med. J . No. 4998, 913 (1956). (14) Ranz, W. E., Wong, ’J. B., Arch. Ind. Hyg. Occupational Med. 5, 464 (1952). (15) Wyld, W., “Manufacture of Sulfuric Acid,” vol. 2, p. 388, Gurney and Jackson, London, 1924. \
I
RECEIVED for review November 27, 1959 ACCEPTEDJune 15, 1960 Previous articles in this series have been published in IND. ENG. CHEM.23, 559. 620 (1931); 27, 587, 659 (1935); 29, 286,1396 (1937) ; 30,101 (1938) ; 32,1037 (1940); 39, 808 (1947); 41, 2403, 2417 (1949) ; 43, 1358 (1951) ; 46,1601 (1954) ; 47, 972, 2426 (1955); 49, 1151 (1957); 50, 1169 (1958). Work is a part of a continuing research program on stack gases in the Engineering Experiment Station of the University of Illinois and was supported in part by a cooperative research contract, No. (1 1-1)27.6 with the U. S. Atomic Energy Commission. Division of Water and Wastes, 136th Meeting, Atlantic City, N. J., September 1959. VOL. 52, NO. 10
OCTOBER 1960
863
