Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2000-07-10
2001-12-25
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S761000, C210S903000, C210S906000
Reexamination Certificate
active
06332986
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 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 are converted to nitrogen.
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 NH4NO3, 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. 4,654,194 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.
BRIEF SUMMARY OF THE INVENTION
This invention relates to an improvement in the wet oxidation of waste streams using ammonium nitrate as the oxidizing agent. The basic process comprises adding ammonium nitrate or precursors thereof to a waste stream in desired amount to reduce the carbonaceous components to carbon dioxide and the nitrogen components to nitrogen. The improvement for reducing the corrosiveness of waste streams contaminated with sulfur or phosphorous containing compounds, whether organic or inorganic, while maintaining reaction rate comprises: operating said process within a pH from about 1.5 to 8 and preferably within a pH range of from about 1.8 to 4.
The basic method for pH control involves maintaining the ratio of M/SO4
−2
of from 0.1 to 4, preferably 0.2 to 1.0 and most preferably a ratio of 0.4 to 0.7. A ratio of M/PO4
−3
of from 0.1 to 2, preferably 0.2 to 0.67 during the wet oxidation process is used. M is an alkali metal or alkaline earth metal cation and the ratio of M/SO4
−2
and M/PO4
−3
is based upon an equivalence basis. By maintaining these ratios, while balancing the reaction such that the carbon species is converted to carbon dioxide and the nitrogen containing species is converted to nitrogen, the corrosiveness of the reaction mixture is reduced and the reaction rate is maintained.
The process of this invention offers several advantages and they include:
an ability to minimize the corrosiveness of wet air oxidation streams by maintaining pH control;
an ability to eliminate the necessity of a second oxidative step to meet total organic (TOC) and total nitrogen removal requirements whereas many prior art processes remove nitrate ion but leave unreacted organics or ammonium ion in the effluent; and,
an ability to maintain excellent reaction rates.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to an improvement in wet oxidation processes involving the destruction of carbonaceous components and nitrogenous components in industrial and municipal wastewater contaminated with sulfur or phosphorus containing components or salts of weak acids and strong bases. The process is a single step wet oxidation process which employs an ammonium nitrate or precursors thereof as the oxidizing agent. Oxygen gas is not required. The process operates in a pH region between 1.5 and about 8 and preferably within a pH region of from about 1.8-4. To accomplish removal of carbonaceous and nitrogenous components to a desirable level requires balancing the oxidation and reduction properties of all of the oxidizable and reducible species present in the wastewater stream. All nitrogen containing species, organic or inorganic, produce substantially only nitrogen and minor amounts of nitrous oxide gas and all carbon containing species produce substantially only carbon dioxide.
The key to pH control and to the maintaining of reaction rate during wet oxidation of wastewater streams contaminated with sulfur or phosphorus substances and alkali and alkaline earth metals (designated M) is in the control of the M/SO4
−2
and M/PO4
−3
ratio (equivalence basis). This is accomplished as follows: contaminants whose anions are of strong acids, e.g., sulfate and phosphates are balanced with alkali or alkaline earth metal cations and conversely, cations of strong bases are balanced with sulfate or phosphate. The ratio of M/SO4
−2
is maintained from 0.1 to 4, preferably 0.2 to 1.0, most preferably from 0.4 to 0.7 and the ratio of M/PO4
−3
of from 0.1 to 2, preferably 0.2 to 0.67 during wet oxidation. Lower ratios, <0.4 for M/SO4
−2
may be tolerated when the process effluent designed permits operation with some residual carbon compounds in the effluent. High ratios reduce reaction rate.
To implement this process and effect balancing of the components in the waste stream, the waste stream is analyzed for composition using well known analytical procedures, e.g., ion, gas and liquid chromato
Burdeniuc Juan Jesus
Johnson Thomas Albert
Leeaphon Malee
Sawicki John Edward
Air Products and Chemicals Inc.
Brewer Russell L.
Hoey Betsey Morrison
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