Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2001-03-21
2002-04-30
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S903000, C210S908000
Reexamination Certificate
active
06379562
ABSTRACT:
BACKGROUND OF THE INVENTION
Wet air oxidation is a well-known treatment process for the removal of COD and BOD from industrial and municipal wastewater streams. The processes involve contacting a wastewater stream with an oxidizing source, such as oxygen, ammonium nitrate and nitric acid at elevated temperatures and pressures to oxidize pollutants. Most carbonaceous material is converted to carbon dioxide. The nitrogen present either from organo-nitrogen compounds or other sources is converted to nitrogen gas.
The following references illustrate wet oxidation processes:
Proesmans, Luan and Buelow of Los Alamos National Laboratory (Ind. Eng. Chem. Res. 1997, 36 1559-1566) report on a high temperature and pressure (500° C./345 bar) hydrothermal oxidation process to remove organic compounds from a waste stream using ammonium nitrate as the oxidizing agent. In the oxidation of methanol and phenol, the authors report that unless an excess of oxidizable carbon is present, NOx in the effluent may become a problem. To avoid NOx production and reduce carbon components to carbon dioxide, a polishing step using hydrogen peroxide is suggested.
GB 1,375,259 discloses the wet oxidation of carbon and nitrogen containing materials to gaseous reaction products using HNO3 and/or a nitrate as oxidizing agent, at temperatures of between 150° C. and the critical temperature of water. The preferred oxidizing agent is NH
4
NO
3
, which disappears completely from the reaction medium. Example VII shows the treating of a waste stream of caprolactam, the sodium salt of aminocaproic acid and sodium sulfate with nitric acid at a temperature of 300° C. at 15 bars. The patentees report that slow heating of the reaction mixture resulted in reduced corrosiveness of the reactant mixture.
U.S. Pat. No. 4,654,149 discloses the use of a noble metal catalyst supported on a titania carrier in a wet oxidation process to decompose ammonium nitrate at 250° C. for 60 minutes. Approximately from 50-99% decomposition of both ammonium nitrate and nitrite is achieved without air present. Further examples show wet oxidation of phenol with 0.2 times the required amount of oxygen.
JP 60-98297, JP 61 257,292 and JP 61 257,291, discloses the catalytic wet oxidation of ammonium nitrate wastewaters with 1.0 to 1.5 times the stoichiometric oxygen required for ammonia decomposition, at a pH of 3-11.5 at a temperature from 100 to 370° C. with a supported noble metal catalyst.
U.S. Pat. No. 5,118,447 discloses a process for the thermochemical nitrate destruction where an aqueous solution of nitrate or nitrite is contacted with a stoichiometric amount of formic acid or formate salt, depending upon the pH. Wet oxidation is effected by heating to 200 to 600° C. in the liquid phase to form elemental nitrogen and carbon dioxide. The reaction may be carried out over a pH range of 0-14.
U.S. Pat. No. 5,221,486 discloses a denitrification process where the types of nitrogen compounds present in a waste stream are identified and quantified. The oxidized and reduced forms of nitrogen are balanced and, then, an appropriate nitrogen containing reactant, such as ammonia or a nitrite or nitrate compound, is added and the mixture is heated to 300 to 600° C. under pressure to effect denitrification.
U.S. Pat. No. 5,641,413 discloses the two stage wet oxidation of wastewater containing a carbonaceous and nitrogen species. In the first stage the COD is removed by wet oxidation at a temperature of less than 373° C. and a pressure sufficient to maintain a liquid water phase. The remaining nitrogen compounds are converted to nitrogen in the second stage by adding sufficient inorganic nitrogen-containing compound to the oxidized wastewater to produce essentially equal concentrations of ammonia-nitrogen, nitrite-nitrogen plus nitrate-nitrogen and a waste stream of reduced COD. Mineral acid is added to the oxidized wastewater to produce a pH between 4 and 7. Optionally, a transition metal salt is added, to catalyze a thermal denitrification step. The last step is conducted at 100° to 300° C. to decompose the nitrogen compounds.
D. Leavitt et al in Environmental Progress 9 (4), 222-228 (1990) and in Environ. Sci. Technol. 24 (4), 566-571 (1990) reported that 2,4-dichlorophenoxyacetic acid, atrazine and biphenyl were converted to CO
2
and other non-harmful gases (N
2
and N
2
O) trough the homogeneous liquid phase oxidation with ammonium nitrate. These reactions were carried out by dissolving the substrates in polyphosphoric acid, adding ammonium nitrate and then heating to about 260° C. for some period of time. Although this clearly shows that ammonium nitrate is a good oxidizing agent, it is not a process lending itself to treating aqueous waste streams containing only 1,000 to 10,000 ppm TOC.
SUMMARY OF THE INVENTION
This invention relates to an improvement in a thermal process for the removal of organic carbon and organic or inorganic nitrogen-containing pollutants from wastewater streams using nitrate salts as the denitrifying agent. The improvement resides in using nitrate salts of aromatic amines (such as those present in the toluenediamine (TDA) waste water) as the main denitrifying source for the dinitrotoluene (DNT) wastewater. To reduce the corrosiveness of the treated effluents, the pH of the liquid is maintained in the range of 1.5 to 8 and preferably from 1.5 to 4 by adjustment with alkali metal.
This process can offer several advantages including:
an ability to work with an influent wastewater having a low pH (1.5 to 2.5) without causing significant corrosion;
an ability to eliminate organic carbon and organic or inorganic nitrogen-containing pollutants in a single step;
an ability to work under strong acidic conditions, e.g., high sulfate, and achieve excellent nitrate removal;
an ability to reduce nitrate and ammonia levels to drinking water standards when working under almost redox balanced conditions in a short period of time (20 minutes);
an ability to reduce nitrate and ammonia levels to drinking water standards when working under almost redox balanced conditions in a short period of time (20 minutes) and in the presence of a relatively high sulfate or phosphate concentrations; and,
an ability to convert most of the organic carbon and organic or inorganic nitrogen-containing pollutants into carbon dioxide and nitrogen gas despite the fact that the operational NH
4
+
/NO
4
−
is much smaller than that required by the stoichiometry (1.66).
DETAILED DESCRIPTION OF THE INVENTION
In our co-pending applications, Ser. No. 09/613,206 having a filing date of Jul. 10, 2000 and Ser. No. 09/659,055 having a filing date of Sep. 11, 2000, the subject mater of each being incorporated by reference, improvements for reducing the corrosiveness of waste streams contaminated with sulfur or phosphorous containing compounds, whether organic or inorganic, while maintaining reaction rate were proposed. The first application proposed: operating said process within a pH range from about 1.5 to 8 and preferably within a pH range of from about 1.5 to 4 by appropriate addition of alkali metal. The second application proposed adding organic material to the waste stream to provide acetate ion in a molar ratio from 0.06 to 0.17 moles per mole nitrate or, in the alternative, should the waste stream contain organic material convertible to acetate in the wet oxidation process, maintain a level of organic material sufficient to provide acetate ion in an amount of at least 0.06 moles per mole of nitrate. The addition, or maintenance of organic material convertible to acetate ion acts as a corrosion inhibitor or buffer assisting in reducing corrosion at pH values of 4 and lower.
However, in the above processes the presence of strong acid anions, nitrate ion removal by ammonium ion becomes more difficult because these acids inhibit the denitrification process. Thus, if a high percent nitrate removal is desired (as expected for effluents that are going to be discharged into the environment), then excess reducing agent or longer reaction times may be required. These
Atr Products and Chemicals, Inc.
Brewer Russell L.
Hoey Betsey Morrison
LandOfFree
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