EMANUEL HURWITZ Metropolitan Sanitary District of Greater Chicago, Chicago, 111. EMlL ClABETTARl Yeomans Brothers Co., Melrose Park, 111.
R. A. WOLFF and IRVING BERNSTEIN Sherwin-Williams Co., Chicago, 111.
Pollution Control by
Air Oxidation of Waste Sulfite liquors
: This research not only solved a difficult problem but
also suggests the type of study that can be used by other investigators confronted wtih similar problems. Many municipal sewage treatment works can be relieved of overload by pretreatment at industrial plants
WASTE
sulfite liquors from the manufacture of chemicals exert a heavy drain on the oxygen resources of aerobic biological sewage treatment works. The Chicago plant of the Sherwin-Williams Co. discharges substantial quantities of these liquors which are derived from their 2-naphthol and p-cresol units. This company, in collaboration with the Metropolitan Sanitary District of Greater Chicago, undertook a joint investigation of the feasibility of oxidizing these liquors before discharge to the Calumet Works of the District. The concentration of sulfite in these liquors ranges from 10 to 20%. I n addition to the sufite, the p-cresol liquor contains dissolved cresols, phenols, and allied material which are thought to be powerful antioxidants (2). T h e 2-naphtho1 liquor is substantially free of substances that strongly inhibit oxidation of the sulfite. The p H of both waste liquors ranges from 7.0 to 9.0, and specific gravity of both wastes is approximately 1.1. The waste liquors from the manufacture of 2-naphthol responded readily to oxidation with air in the presence of a cobalt chloride catalyst. Waste liquors from p-cresol manufacture and mixed p-cresol and 2-naphthol liquors were more difficult to oxidize because of extremely water-soluble antioxidants formed in the manufacturing process. The optimum K,a for a Mixco system was 0.08 when a cobalt chloride catalyst was used. The optimum Koa for a Yeomans cavitator system was 0.04. The horsepower required to oxidize one pound of sodium sulfite in the 2-naphthol liquor averaged 0.018. I n full size units,
better efficiencies can be expected from both types of systems.
Experimental T o obtain a high degree of control and repeatability, laboratory M,ere conducted as a batch operation (Figure 1). The sulfite liquor was pumped to the top of a packed column and discharged into a reservoir at the bottom of the tower. The oxidizing gas flowed countercurrent to the sulfite liquor. v
so exiernal heat was supplied to replace that produced by the exothermic airsulfite reaction which was lost to the atmosphere. Temperatures were held
GLASS RINGS GLASS TUBE FLOW METER
40”
I-
THERMOMETER
I_ co N \ rpUMp
BEAKER POWERS
TR OL
J Figure 1.
Laboratory tests were conducted as a batch operation Liquid flow 3 iiterr/min. Air flow 0.1 6 cu. ft./min.
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at 60" f 2" C. Liquid flow was 3 liters per minute and gas flow 4.5 liters per minute. Rate of oxidation was not significantly affected by p H variations in the range found in the p-cresol or 2-naphthol liquors ( 7 , 3, 4,6). Therefore. no adjustment for p H was made either before or during the oxidation process. Results are reported as the slopes of curves obtained by plotting the change in sodium sulfite concentration against time. As other variables-gas and liquid flow and temperature-are held constant, the relation between concentration and time is linear. Pure Oxygen. A series of exploratory tests was made with pure oxygen as the oxidizing gas to observe the effect of catalysts on the oxidation rate of sulfite and the response of a composite sample of p-cresol and 2-naphthol liquors to oxidation with and without catalysts. Copper sulfate had no catalytic effect on the oxidation rate of pure sodium sulfite (Table I). Cobalt chloride produced a marked acceleration in the oxidation rate of both pure sodium sulfite and the composited plant liquors. However, the oxidation rate of the plant liquor was much lower than that of pure sulfite, suggesting the presence in the plant liquors of substances with antioxidant properties. Phenolic substances are known to be such inhibitors, but because plant liquors. stripped of phenols down to 0.0270 by steam, showed no appreciably increased response to oxidation the presence of other antioxidants was suspended. There is present in the cresol liquors a minute quantity of a tvater-soluble dihydroxy compound which exhibits very strong antioxidant properties. It is this substance which is believed to be responsible for retarding the rate of oxidation of the composited plant liquor. Air as Oxidizing Agent. Using atmospheric air as the oxidizing gas, studies \vue next directed toward the oxidation of the p-cresol waste liquor (Table 11). As already shown, this material contains powerful antioxidizing substances and also contains sodium sulfate in about the same concentration as sodium sulfite, 11%. The effect of sodium sulfate on the rate of osidation is reported iri Table 111.
Table I. Cobalt Chloride Accelerated Oxidation of Sulfite Wastes with Pure Oxygen Rate Liquor Catalyst (Slope) C.P. NanSOa C.P.
Na~S03
C.P. NarSOa
Composite (plant) Composite (plant)
1 302
None cuso4 COCl? None CoClz
0.217 0.213 0.427 0.095 0.293
VARIABLE SPEED D R I V E
i BAFFLE (4) FOAM LEVEL
HOTAME TER S FEED
FEED PUMP Figure 2. In the Mixco system, air or gas is introduced through a sparge ring and dispersed into the liquid phase by a mechanical agitator
I n high concentrations sodium sulfate lowers the oxidation rate of pure sodium sulfite (Table 111). This effect is decreased by dilution. This would suggest that the reduced rate of oxidation is caused by either or both of two factors-i.e., the loww solubility of oxygen in the sulfate-laden liquor, or a reduction in the number of contacts between the sulfite and oxgen molecules because of interference by the sodium sulfate. Waste cresol liquor, stripped of phenolic substances. )vas diluted 1005x
Table II. Dilution Increased Oxidation Rate Somewhat, but Dihydroxy Compounds Had an Inhibiting Effect
and 300%, Lvith ivater. The oxidation rate remained l o ~ vsuggesting the presence of a potent water-soluble inhibitor. This was partially identified by infrared spectrophotometry as a dihydrosy compound. \Vhen extracted frorn the cresol liquor. and added to pure sodium sulfite the rate of oxidation was lowered to 0.042 from 0.090. It is knobvn that 4methylresorcinol could be formed as an intermediate product in the manufacture of p-cresol. LVhen this compound was added to pure sodium sulfite in concentrations of 0.057c, the rate of osidation \vas lobyered from 0.090 to 0.012, indicating that it or a similar dihydroxy compound \vas at least partly responsible for the difficulty encountered in the oxidation ofp-cresol.
10 p.p.m. CoCl:
Liquor Trentiiient 1 1 % C.P. NaZSOa, 4-methylresorcinol Cresol liquor, 100% dilution Cresol liquor, 300% dilution 11% .c.P. NazS03, unknown inhibitor 1 1 % C.P. NazSOa
INDUSTRIAL AND ENGINEERING CHEMISTRY
Rate (Slope 1
Table 111. Sodium Sulfate Lowered the Oxidation Rate 10 p.p.m. CoCIz
0.012 0.024 0.034 0.042 0.090
Liquor
.\ddltln?
Ilatr (SlOl?P1
11% Na2S03
None
0.090
11% Na~S03 5.57, NarSOa
1 1 % NazSOI
0.030
5% Nays04
0.070
A I R OXIDATION O F SULFITE L I Q U O R S Table IV.
All the Surfactants Worked Well
Table V. Surf Seemed to Supplement Surfactants in the Liquor
Cresol liquor containing 10 p.p.m. CoCIz
Treatment
No catalyst, no dilution No dilution Pretreatment with activated carbon and 100% dilution 100% dilution plus 100 p.p.m. NaaPOd 100% dilution plus 20 p.p.m. Pluronic 100% dilution plus 20 p.p.m. Surf
Rate (Slope) 0.017 0.028 0.038
a
100% dilution; 10 p.p.m. CoClz Surf Added, P.P.M. Rate (Slope) 0 0.087 20 0.110 20' 0.137 Repeat experiment.
0.058 0.040 0.052
It had been observed that the 2naphthol wastes responded readily to oxidation and in some tests the rate of oxidation was greater than that of pure sulfite. T h e 2-naphthol wastes contain the sodium salt of 8-naphthalenesulfonic acid which acts as a wetting agent. Taking a clue from this, a number of surfactants were added to a new type of p-cresol wastes which was extremely resistant to oxidation. All the surfactants gave encouraging results (Table IV). Of these Surf was the most effective. Surf was added to diluted composite liquors and its effect observed. An appreciable improvement was noted (Table V). The detergent appears to supplement the surfactants occurring naturally in the 2-naphthol liquor. These laboratory experiments indicated that cobalt chloride is an effective catalyst for accelerating the oxidation of sulfite liquors; dissolved salts depress oxidation; dilution increased the rate of oxidation by counteracting the effect of dissolved salts ; naturally occurring inhibitors, such as certain dihydroxy compounds, reduce the rate of oxidation by preventing the efficient utilization of dissolved oxygen ; and surfactants accelerate the transfer of oxygen by lowering the interfacial tension between the gas and liquid. The positive effects of catalysts, dilution, and surfactant are roughly additive.
propels them at high velocities into the liquid. This action disrupts stagnant films at the gas-liquid interface effecting the transfer of the gas to the liquid. The speed and design of the impeller are critical factors effecting the efficiency of the unit. The Yeomans cavitator unit consisted of a wooden vessel 2 feet square and 15 inches deep (Figure 3). Aeration is accomplished by establishing regions of cavitation with a rapidly revolving rotor
moving through the liquid. Air is drawn into these cavities, by the vacuum created in them, through a hollow shaft and hollow rotor. T h e cavities assume the form of very fine bubbles which continue to maintain their bubble structure until the temperature and pressure within them reach a state of stability. At this point the vapor condenses and the bubble collapses with considerable shock, disrupting stagnant surfaces and exposing them to aeration. The rotor also creates a pumping action, causing the liquid to be thoroughly mixed throughout the tank by circulating it through a draft tube. High absorption efficiencies are attained by the rapid exposure of fresh surfaces to both bubble and surface aeration. Two parameters were used to evaluate efficiency of these units, the gas transfer coefficient, Koa, and the power consumed per pound of sodium sulfite. The Koa is a value which expresses
Semi-pilot Plant Studies
Semi-pilot plant investigations to determine the practicability and economy of oxidizing these wastes on a continuous basis were made using an aerator designed by Mixco (Mixing Equipment Co., Rochester, N. Y . )and a Yeomans cavitator system (Yeomans Brothers Co., Melrose Park, Ill.). The Mixco equipment consisted of a vertical, stainless steel, baffled vessel, 38 inches in diameter and 6 feet deep (Figure 2). Air or gas is introduced through a sparge ring and is dispersed into the liquid phase by a mechanical agitator. The rapidly rotating impeller shears the gas into tiny fragments and
UECC((IWL.ir 5 ou LIrJL
Figure 3. Aeration in the Yeomans unit is accomplished b y establishing regions of cavitation, in the form of very fme bubbles, with a rapidly revolving rotor moving through the liquid VOL. 51, NO. 10
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Table VI.
Feed,
Test NO.
Gal./Hr. 1.8 19.2 19.2 19.2 25.2 25.2 66.0 66.0
a
Efficiency of the Mixco Unit for Oxidation of Waste Sulfite Liquors
Sulfite Concn., Influent Effluent 13.6 13.2 13.4 13.6 10.4 10.4 9.6 9.1
3.2 0.77 1.00 2.20 2.60 2.80 0.12 0.10
Input. c'u. Ft./
.iir Efficiency,
7.5 47.0 25.0 25.0 33.0 33.0 33.0 33.0
2.52 5.26 10.00 9.15 6.20 6.05 19.7 18.8
Miti.
( n /C
Horsepower Requirements -__
Sulfite
Horsepower/
Sulfite Oxidized
fim
0.713 0.111 0.116 0.050 0.069 0.055 0.016 0.015
0.0236 0.0207 0.0163 0.0135 0.028 0.0274 0.0869 0.0764
oxidized
Compresora
Mixer
Total
per Hr.. Ltxh
0.28 0.96 0.50 0.50 0.70 0.67 0.70 0.70
0.95 1.46 2.04 0.53 0.60 0.30 0.23 0.12
1.23 2.42 2.54 1.03 1.30 0.97 0.93 0.82
1.72 21.8 21.8 20.5 18.0 17.5 57.2 54.3
1,h.
Based on quantity of air required t o reduce sulfite. H o r - e ~ m ~ required r t o fiirnisli air taken from I ~ i ) [ ) t ~ - ( ~ ( ~ i l t i tnble,. ~ ~ ~ ~ i l l i Sp. , gr. = 1.1. 2-Naphthol base liquor, no r a t a l y t . 2-Naphthol mixed liquor, n o c n t a l p t . e 3-Na~)ht,holmixer1 liquor, 1 0 p.p.rn.
sulfite liquor coc12.
Table VII.
Feed, Gal. IHr.
Sulfite
Efficiency of Yeomans Cavitator for Oxidation of Waste Sulfite Liquors
% Effluent
Cu. Ft.
Sulfite .Air Oxidized l
