Oxidation of wastewaters

The present invention relates to an improved process for the wet oxidation of water soluble organic pollutants or of an aqueous suspension of organic pollutants. In the contacting of an oxidizing gas and a polluted aqueous phase, the improvement comprises forming a fine mist of the polluted aqueous phase in the presence of the oxidizing gas, thereby increasing the interfacial area between the gas and the polluted aqueous phase. Then the formed mist is introduced into a heated reaction chamber under pressure, thereby enhancing the rate of the destructive oxidation of the organic pollutant by the increase in mass transfer between the gaseous phase and the aqueous mist, the reaction temperature being selected to favor rapid destruction of the pollutant without the formation of stable intermediate reaction products. After destruction of the pollutants, the reaction mixture is allowed to flash off at a pressure lower than the reaction pressure.

BACKGROUND OF THE INVENTION 
Phenolated and hydroxylated aromatic compounds are one of the main sources 
of industrial pollution. 
Phenolated residual water is found in the effluents of industries involved 
in the manufacturing of pharmaceutical products, plastic materials, coals, 
tars and their derivatives, pesticides and dyestuff among others. 
The residual phenol concentrations vary widely, depending on the type of 
industry involved. These concentrations may attain several grams per liter 
and since it is known that phenols are toxic to living organisms even at 
very low concentration levels, it has become necessary to develop 
purification techniques for treating phenolated wastewaters. 
However, these techniques have not been very numerous, have almost always 
involved substantial investments, and, above all, they have not been 
totally effective. So far, the most effective way to dispose of soluble or 
suspended organic pollutants in aqueous systems has been to chemically 
oxidize the aromatic contents either totally to carbon dioxide or 
partially to acids which are easily degradable by further action or 
microorganisms. 
In the light of the numerous studies performed on the oxidation of 
phenolated wastewaters, it can be concluded that there are two key aspects 
which have to be looked upon. They are the chemical steps leading to 
destruction of the toxic soluble organic material and the configuration of 
the reactor system in which contacting between liquid and gas phases is 
made. 
The chemical steps leading to oxidation of aromatic compounds are 
relatively well understood. Basically, oxidation is initiated by the 
formation of hydroperoxide radicals leading to hydroquinones and quinones 
and followed by further ring opening and destruction of the aromatic 
structures. 
Since oxidation is undoubtedly the most effective treatment of phenolated 
wastewaters, many variations of this method have been developed. It is 
clear although that a flexible and inexpensive purification process has 
long been sought after, and numerous publications attest these facts. 
It has been proposed to effect oxidation treatment by ozone or 
permanganate. However, these two products are extremely costly and the use 
of permanganate results in the production of large quantities of 
undesirable sludge. 
Treatment by chlorine has also been considered to be interesting, but it 
frequently produces toxic chlorophenols and this opposes the achievement 
of the desired aim, which is precisely to avoid the formation of such 
undesirable intermediates. 
Oxidation using hydrogen peroxide mixed with a salt of ferrous iron as 
catalyst, conventionally known as the Fenton reagent, has also been 
proposed and this process was found to be among the most effective ones. 
However, it presents some disadvantages, namely the necessity of 
introducing ferrous iron which must be separated after processing, acid pH 
that is strong enough to attack the reactor walls, very high production 
costs and finally hydroxylation of the hydrocarbides which may be 
contained in the wastewater to be purified. 
The concomittant use of UV light, temperature and acoustic energy to 
trigger the free radical oxidation mechanisms has also been reported. 
Finally, direct wet air oxidation using HSO.sub.5 as a catalyst has been 
reported and applied to the oxidation of toxic phenolic compounds in 
wastewaters. 
In the use of gaseous oxygen with or without a catalyst, the contacting 
between the wastewater and the oxygen containing gaseous phase is almost 
always effected by bubbling the gas through the liquid using a variety of 
agitation systems. However, it will be understood that mass transfer 
limitations are encountered in current technologies since the gaseous 
oxygen has to diffuse through the gas-liquid interface using the 
inherently low external surface area available in the gas bubbles. Low 
oxidation rates are thus obtained necessitating long treatment times. This 
results in massive technologies having significant investing and operating 
costs. 
Thus, in the light of existing technology, it would be highly desirable to 
provide a new method for treating contaminated wastewaters without leading 
to undesirable stable reaction intermediates that would be rendered more 
efficient by improving mass transfer between the contaminated waste and 
the oxidizing gas. 
SUMMARY OF THE INVENTION 
The invention is related to an improved process for the wet oxidation of 
water soluble organic pollutants or of an aqueous suspension of organic 
pollutants. In the contacting of an oxidizing gas and a polluted aqueous 
phase, the improvement comprises forming a fine mist of the polluted 
aqueous phase in the presence of the oxidizing gas, thereby increasing the 
interfacial area between the gas and the polluted aqueous phase. Then the 
formed mist is introduced into a heated reaction chamber under pressure, 
thereby enhancing the rate of the destructive oxidation of the organic 
pollutant by the increase in mass transfer between the gaseous phase and 
the aqueous mist, the reaction temperature being selected to favor rapid 
destruction of the pollutant without the formation of stable intermediate 
reaction products. After destruction of the pollutants, the reaction 
mixture is allowed to flash off at a pressure lower than the reaction 
pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the filtered wastewater feed stream is pumped 
through a line 10 which can act as a preheater via indirect heating, 
before mixing with the compressed oxidizing gas coming through another 
line 20 takes place into an injector 6. The gas-liquid mixture goes to a 
tubular reaction chamber 4, where it is introduced in the form of a fine 
mist and rapidly heated up to a temperature ranging from 140.degree. C. to 
200.degree. C. at pressures ranging from 2 to 4.5 MPa for a prescribed 
period of time ranging from 0.1 to 3 minutes. The tubular reactor 4 can be 
heated either indirectly or by a live steam addition system 14. The outlet 
of the reaction chamber 4 then goes through a line 40 to a flash drum 6 in 
order to bring the system down to the chosen discharge pressure and 
temperature. The flashed steam is recovered through a valve 16 whereas the 
treated wastewater leaves the system through another valve 8. Recycle 
loops or a series of injector-reactors are possible depending upon the 
severity of the treatment chosen. Carbon dioxide is then the only 
contaminant of the steam since total oxidation of the organic matter has 
taken place. 
Moreover, it is important to note that the addition of a suitable liquid 
catalyst such as H.sub.2 O.sub.2 to the reaction system can result in 
considerable increase of the reaction rates, that being due to an 
energetically more formable initiation path. The effect of the catalyst on 
the reaction rate can be easily visualized by comparing the results shown 
in Table I. 
It is also to be noted that an important feature of this invention is that 
large bubbling tanks commonly used for wastewater treatment have now been 
replaced by a compact reactor that can be transported to the wastewater 
storage site, thus avoiding unnecessary transportation of hazardous 
chemicals. 
However, the main features of the present invention remain the improved 
contacting between the oxidizing gas and the organic pollutant which 
provides for excellent waste destruction at low costs and the absence of 
formation of undesirable intermediates or introduction of undesirable 
substances. 
Thus, it has been appreciated that a striking advantage of the present 
invention is that it provides for the efficient elimination of phenolic 
compounds to a concentration ranging from between 10 to 30 mg/l in a cost 
efficient manner and by a portable apparatus. The phenol concentration of 
10 to 30 mg/l is an acceptable level by environmental regulations for 
disposal of such wastewaters in sewers. 
After that 10 to 30 mg/l level has been reached, a second purifying 
technique such as activated carbon or biological treatment either of which 
is usually provided by municipalities for the treatment of sewage waters 
can then be used to remove the remaining phenolic compounds. It is to be 
reminded that activated carbon even though it is very efficient can only 
be used for treating low phenolic concentration. This is the reason why 
other techniques like the techniques of the present invention need to be 
implemented for the treatment of higher phenolic concentrations. The cost 
of using the combined techniques involves costs sharing by both the 
industry, which is reducing the high phenolic contents of its wastewaters 
down to concentrations lower than 30 mg/l and the municipality which is 
treating the low phenolic concentration wastewaters in the conventional 
water treatment plant. These features constitute a major step forward as 
far as organic waste disposal is concerned. 
The present invention will be more readily understood by referring to the 
following Examples which are given to illustrate rather than limit the 
scope of the invention. 
EXAMPLE 1 
An aqueous solution containing 1000 mg/l of phenol was pumped at a rate of 
0.7 l/min. using a MH32C high pressure pump, preheated using an 
electrically heated tubular heat exchanger, and then injected into an 
injector/mixer having a central jet orifice of 0.016 inch and two 
peripheral orifices of 0.035 inch for oxygen introduced corresponding to a 
multiple of the stoichiometric amount needed to oxidize the phenol. The 
intimately mixed gas/liquid phase was then introduced into a tubular 
reaction chamber (volume 0.86 land diameter 0.5 inch) which had an 
internal temperature of 145.degree. C. and an internal pressure of 2.6 MPa 
for a period of time lower than three minutes. After treatment, the 
solution was flashed via a fixed orifice into a flash drum reservoir where 
it was immediately cooled to 100.degree. C. Steam and non-condensible 
gases were then released and steam was later condensed. The resulting 
liquid was then analyzed by chromatography. Results are shown in Table I. 
EXAMPLES 2-4 
The same procedure as in Example 1 was followed, the only modification 
being the internal reaction chamber temperature, which was respectively 
maintained at 160.degree., 170.degree. and 180.degree. C. Results are 
shown in Table I. 
EXAMPLE 5 
The same procedure as in Example 1 was followed using a reaction chamber in 
which a solid CaO/Cr.sub.2 O.sub.3 catalyst (Harshaw, 3.5% CuO, 38% 
Cr.sub.2 O.sub.3, 10% BaO in 1/16 inch pellets) was embedded. However, the 
catalyst performed poorly, even lowering the conversion rates obtained 
through direct oxidation. This lowering could be due to a decrease of the 
interfacial area between gas and liquid droplets caused by rapid 
coalescence of the mist when in contact with the catalyst bed. Thus, the 
poor conversion rates observed with the solid catalyst tend to confirm 
that the reaction is taking place in the liquid phase. 
EXAMPLES 6-7 
In Examples 6 and 7, the same procedure as in Example 1 was repeated on 
phenolic aqueous solution containing hydrogen peroxide at a concentration 
of 9.8.times.10.sup.-3 mole/l. The internal reaction chamber temperature 
was maintained at 170.degree. for Example 6 and 180.degree. C. for Example 
7. As it can be seen in Table I, a higher conversion rate was observed at 
170.degree. C. It could be speculated that at 180.degree. C., fast 
decomposition of the hydrogen peroxide occurs, thus leading to lower 
rates. 
TABLE I 
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Phenol conversion times at different temperatures with and without a 
H.sub.2 O.sub.2 catalyst 
Initial phenol 
Initial H.sub.2 O.sub.2 
Conversion Final phenol 
(mg/l) (mol/l) 
(.degree.C.) 
Conversion time 
(mg/l) % Phenol 
Example 
concentration 
concentration 
temperature 
(min.) concentration 
converted 
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1 1000 0 145 97.3 10 99 
2 1000 0 160 37.8 10 99 
3 1000 0 170 26.3 10 99 
4 1000 0 180 17.9 10 99 
6 1000 9.80 .times. 10.sup.-3 
170 16.9 10 99 
7 1000 9.80 .times. 10.sup.-3 
180 11.9 10 99 
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