Catalytic Oxidation of Sulfite Waste Liquor by Atmospheric Oxygen IRWIN A. PEARL1AND HENRY IC. BENSON University of Washington, Seattle, Wash.
ULFITE waste liquor exerts a considerable oxygen demand when discharged into a large body of water. The initial or immediate oxygen demand is a chemical demand upon the dissolved oxygen of the water by the free and loosely combined sulfur dioxide of the waste liquor. This demand is instantaneous and approaches completion shortly after contact of the liquor with dissolved oxygen. The reaction is complete only when either the dissolved oxygen or the oxidizable sulfur compounds are consumed. The initial or immediate demand is followed by a gradual oxygen demand which is probably due to the biochemical oxidation of the easily oxidizable organic matter such as sugars, alcohols, acids, etc. The final oxygen demand is that due to the slow biochemical oxidation of the lignin residue in the liquor. From a careful inspection of a large number of analyses of Puget Sound water in the vicinity of a pulp mill, it was found that when sulfite waste liquor is discharged into sea water, the immediate or chemical demand is the only oxygen demand of consequence. Once the chemical demand is satisfied, the slower biochemical demands can easily be satisfied by the sea water, owing to its normal processes of extracting oxygen from the atmosphere and phyto-organisms. A similar conclusion was reached by Schwabe and Lagally (7) who stated that the rate of oxygen consumption is so slow as to constitute no serious problem in river water, and that the surrounding air will always be able to supply more oxygen than that consumed in the corresponding period. This conclusion is further substantiated by results of an experiment in which variations of the oxygen demand with time were studied. The dissolved oxygen was determined every 15 minutes on a sample of oxygenated water which was treated with digester-strength waste liquor. Figure 1 gives the results obtained when 10 ml. of digester liquor were added to 2500 ml. of oxygenated water having a dissolved oxygen content of 22.6 mg. per liter. Corresponding results were reported by the Wisconsin State Board of Health (Q),which determined the oxygen demand of sulfite waste liquor as a function of time. These data clearly indicate that the immediate or chemical demand is the only demand of immediate consequence, and that even a slow rate of absorption of oxygen would be adequate to satisfy the slow requirements of the biological oxygen demands. Because of this fact and because the immediate chemical oxygen demand is due to the Oxidizable inorganic sulfur compounds. a search was made for a catalyst to make possible the oxidation of the latter substances in the waste liquor by air prior to its discharge into a waterway.
When sulfite waste liquor is discharged into sea water, the immediate chemical oxygen demand is the only oxygen demand of consequence. Activated carbon is an effective catalyst for the oxidation of the chemically oxidizable compounds in sulfite waste liquor by aeration. Activated carbon reduces foaming in sulfite waste liquor aeration processes, and its activity does not decrease with use in these aerations.
S
It is well known that solutions of sulfites, bisulfites, and sulfur dioxide are unstable toward atmospheric oxidation and are rapidly oxidized to solutions of sulfates. It is equally well known that the presence of even small quantities of organic matter such as sugars in solution will retard this oxidation to such a degree that a solution of a sulfite in the presence of sugar will remain unchanged for months. I n the case of sulfite waste liquor the presence of organic matter in solution prevents any oxidation from taking place when digester liquor or its concentrated solutions are aerated with either air or oxygen. However, dissolved oxygen rapidly oxidizes the sulfites even in the presence of the organic matter. Aeration Studies Several investigations of the effect of aeration of sulfite waste liquor upon the B. 0. D. have been reported (6, 9, IO). Considerable losses of total B. 0. D. were reported, but in all cases work was performed upon the diluted liquor, and no recognition was made of the immediate chemical oxygen demand. This well known concept of immediate chemical oxygen demand was utilized in the following studies on catalysts for the atmospheric oxidation of sulfite waste liquor. The waste liquor used was a digester liquor having a total solids content of 132.1 grams per liter. As noted above, it was found that a t room temperature the aeration of 10 per cent digester liquor in a tower by introduction of the air through a Carborundum block a t the base of the tower had no effect upon the sulfite content of the liquor. Small quantities of sulfite oxidation catalysts such as copper sulfate, ferrous sulfate, cobalt sulfate, manganese sulfate, and nickel sulfate (2,3, 4, 6, 8) were added, but they had no effect upon the oxidation of the sulfites in the waste liquor. Aeration through spray nozzles was then tried, but the results were the same. The temperature of the solutions was then raised and the experiments were repeated. Results for the Carborundum block method are shown in Figure 2. Similar results were obtained by spray methods. These data indicate that a t temperatures above room temperature the sulfite content is reduced somewhat, but not entirely, by aeration.
>Presentaddress. Institute of Paper Chemistry, Appleton, Wis.
436
April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
The aeration tower was then filled with Alfrax porous pellets which had been impregnated with a number of catalysts. Ten per cent digester liquor was introduced dropwise upon the top of the column countercurrent to air introduced a t the base of the column at the rate of 1.2 cubic feet per hour. The apparatus was constructed so that the aerated waste liquor drained a t the bottom. The immediate chemical oxygen
TIME
-
HOURS
FIQURE1. VARIATION OF OXYQEN DEMAND WITH
TIME
437
0. D. could be readily oxidized to sulfates by the oxygen adsorbed on activated carbon particles. (The activated carbon used in all of these studies was a product supplied by the Carlisle Lumber Company, of Seattle. The data and conclusions in this paper refer only to this particular carbon.) Waste liquor samples were easily oxidized by activated carbon granules to the limit of their adsorbed oxygen. I n this case the oxygen adsorbed on the active carbon reacted with the oxidizable sulfur compounds in the same manner as does dissolved oxygen. I n the presence of activated carbon the chemically oxidizable portion of sulfite waste liquor could readily be oxidized by aeration, and after the immediate chemical demand was satisfied, the liquor could be given a dissolved oxygen content equal to or exceeding that of natural waters. The exact mechanism of this reaction is not clear. The catalytic behavior of the activated carbon in this process does not change with time as does the activation or absorbing capacity of activated carbon in other water purification processes. The carbon appears to act as a carrier for oxygen and seems to selectively absorb the oxygen and then relay it in an active state to the chemically oxidizable substances in the waste liquor; these substances are thereby transformed to the stable sulfates. Once these oxidizable substances are gone, the excess of air easily saturates the waste liquor with oxygen (air).
Wne dotted line is the original dissolved oxygen
demand (I. C. 0. D.) was determined on the aerated liquor after each run by adding a known volume of the sulfite waste liquor or one of its dilutions to a large known volume of oxygen-saturated water whose dissolved oxygen value was known. The Winkler reagents were added immediately and the dissolved oxygen was determined. From the two dissolved oxygen values the I. C. 0. D. was calculated and reported as mg. of oxygen per liter of waste liquor. Dissolved oxygen values were determined by the unmodified Winkler procedure (1). This method was checked against the alkaline hypochlorite modification (1) and showed little difference. Because of the greater ease of manipulation the unmodified method was chosen for the I. C. 0. D. determinations. The effluent from the first run was reintroduced at the top for the second run, etc. Results of these experiments are shown in Table I. They show that by aeration in the presence of certain catalysts the I. C. 0. D. may be materially reduced. However, the acid solution dissolves the oxide catalysts and soon the catalysts disappear from the porous pellets. Neutralization of the liquor might solve this problem, but the foaming tendency is increased many fold. I n these experiments the loss in I. C. 0. D. might have been due to chemical oxidation of the sulfites by the oxidized catalysts. However, repeated runs were not made to clear up this point.
TIME
-
HOURS
FIQURE 2. AERATIONOF 10 PERCENTSULFITE WASTELIQUORAT ELEVATED TEMPERATURES
It was found that the presence of activated carbon almost eliminates the foaming which always accompanies aeration of sulfite waste liquor and its dilutions. This fact makes it possible to use much higher air velocities in aeration processes. Aeration Methods Aeration of sulfite waste liquor and satisfaction of its I. C. 0. D. was accomplished by two methods illustrated by
TABLEI. RESULTS OF AERATIONTESTS 7
Catalyst Pt
Digester Liquor, % 100
Fe:Os CrtOs
10 10 100
CUO MnOr
10 10
10
Run 1 1327 134 94 1310 41 95 54
Mg. per Liter I. C. 0. D. Run 2 Run 3 1327 1327 134 134 94 94 1148 977 22 14 81 72 0 0
Control 1327 134 94 1397 131 135 134
, I n the course of the search for an oxidation catalyst it was noted that the oxidizable inorganic sulfur compounds of sulfite waste liquor which are responsible for the latter’s I. C.
the following experiments: EXPERIMENT 1. An u right cylindrical column 10 feet in height and 2.5 inches in Jameter was loosely packed with 6-8 mesh granules of activated carbon to a depth of 6 feet. The column was then charged with 1 gallon of 10 per cent digesterstrength sulfite waste li uor which filled the column t o a depth of approximately 8 feet. ?he waste liquor had an I. C . 0. D. of 135 mg. of oxygen per liter of waste liquor. Air was introduced at the base of the column at the rate of 2 cubic feet per hour. No foaming occurred. After a time la se of 5 minutes the I. C. 0 D. had disappeared, and the waste Hquor gave a positive test for dissolved oxygen. After 10 minutes the liquor had a dissolved oxygen content of 14 mg. per liter, which is higher than the concentrations of dissolved oxy en usually reached by water in our natural waterways. The sulgte content of the liquor was reduced t o zero.
INDUSTRIAL AND ENGINEERING CHEMISTRY
438
EXPERIMENT 2. The column used in experiment 1 was filled in the same manner with activated carbon and sulfite waste liquor. After aerating for 10 minutes, waste liquor was added at the rate of 250 cc. per minute at the top of the column, and the aerated liquor was drained at the base of the column at the same rate. In this way the process became continuous. In both experiments 1 and 2 the carbon particles were first thoroughly washed with a large volume of waste liquor to ensure their being free from adsorbed oxygen. It was found that carbon particles of 6-8 mesh size were best suited for this process. Smaller particles tended to pack in the column and larger particles were not so efficient. The activity of the carbon did not decrease with time, and the carbon showed the same catalytic power even after use for a period of 5 weeks.
Acknowledgment The sulfite waste liquor used in these experiments was of hemlock origin and was kindly furnished by the Puget Sound Pulp and Timber Company of Bellingham, Wash.
Vol. 34, No. 4
Literature Cited Standard Methods of Water Analysis,
(1) Am. Pub. Health Assoc., 1936.
( 2 ) Fink, C. G., Trans. Electrochem. Soc., 71, 39 (1937). (3) Milbauer, J., and Pazourek, J., Bull. SOC. chim., 31, 676 (1922). (4) Milbauer, J., and Pazourek, J., Chem. Listy, 15, 35 (1921). ( 5 ) O'Dell, M.J., and Greenlaw, A. X., Paper Trade J . , 99, No. 8, 41 (1934). (6) Reinders, W., and Vles, S. I., Rec. truv. chim., 44, 249 (1925). (7) Sohwabe, K., and Lagally, P., Wochbl. Pupiwfubr., 69, 1037 (1938). (8) Volfkovich, S. I., and Belopolskit A. P., J . Applied Chem. (U.5. S . R.),5, 509 (1932). (9) Wisconsin State Board of Health, Bur. of Sanitary Eng., Rept. on Stream Pollution in Wis., 1927. (10) Wisconsin State Board of Health, Rept. on Pollution Survey at Green Bay, Wis., 1938-39 (1939). PRESENTED before the Division of Water, Sewage, and Sanitation Chemistry at the 102nd Meeting of t h e AMERICANCHEMICAL SOCIETY, Atlantic City, N . J.
Densitv and Porositv of Carbonaceous Materials J
J
R. C. SMITH, JR., AND H. C. HOWARD Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Penna.
I
N T H E classical method of determining the density of a solid substance by placing a known mass of it in a pycnometer and displacing the air by a liquid, the results, in the case of finely porous structures, may vary significantly with different liquids. Evacuation before admission of the liquid, or boiling of the liquid, assists in displacement of gases; but with such fine structures as are found in activated carbons, for example, densities with different liquids varying as much as 15 per cent have been reported ( 2 , 7 ) ,even after good evacuation. One group of workers (2, 9, 12) has ascribed the observed variations in the case of active carbons to different degrees of penetration of the pore structure by the various filling fluids, another (1,6, 7 , 21),to different degrees of compression of the fluids by adsorption a t the solid-liquid interfaces. On the basis of the first hypothesis, the highest density observed will be the most nearly correct; from the latter, one must conclude that the higher results are fictitious, due to compression of the filling fluid to greater than normal density. A recent study (3) of the densities of active carbon and of silica gel emphasized the importance of wetting in determining the densities observed in different liquids. While compression of liquids a t liquid-solid interfaces, of sufficient magnitude to cause significant errors in a density determination, has not been experimentally demonstrated, there is no doubt that gases may be held in a compressed state on solid surfaces and consideration must be given to this factor whenever gaseous pycnometer fluids are used. Adsorption measurements on active carbon surfaces have shown that helium, as would be predicted from its properties, is the least adsorbed of all gases. The older data of Dewar (5),with helium of unknown purity, indicate some adsorption a t 0" C., and Homfrsy's (8),none at all a t room temperature. More recent work (9,19) has shown that adsorption is negligible a t room
temperatures and gbove, since the density of activated carbon determined with helium gas as a filling fluid was found to be unaffected by changes in either the temperature from 25' to 75" C. or the pressure of the helium. Helium gas has been used as the filling fluid in density determinations of ceramic bodies (20),various forms of carbon (9), coal (15), cotton (Q), and wood (16).
Experimental Method The apparatus employed in the Coal Research Laboratory since 1932 is shown in Figure 1 and is a modified form of that used for measuring adsorption isotherms (14). The method consists in introducing a known volume of helium gas, a t known temperature and pressure, into a pycnometer of known volume containing a weighed sample, and measuring the new temperature and pressure of the gas. From these data the volume of the free space, the volume of the sample, and the density of the sample can be readily calculated. The size of the apparatus is indicated by graduated scale Q which is 125 cm. long. It consists of a pycnometer, U , of known volume to the mark TI which is detached for filling by cutting the tubing between T and the base of the thermostat and which, after filling, is resealed into position; a flask of helium, A; a Toepler pump, K ; a measuring bulb, 0, of known volume, connected to the calibrated capillary tube, PR, which in turn is supported next to the graduated scale, and a McLeod gage, Y . Evacuation of the entire apparatus is accomplished through tubes F and &, which connect with each other and lead to a trap, a mercury diffusion pump, a large bore stopcock, and a Hyvac pump. Thus, both sides of the apparatus can be connected to the vacuum pump simultaneously. A common suction manifold (not shown), served by a water aspirator, is
