Title of Invention

"A METHOD FOR TREATING ALKALINE WASTEWATER"

Abstract A method for treating alkaline wastewater comprising subjecting alkaline wastewater containing a high concentration of at least one species selected from the group consisting of nitrogen compounds, organic substances and inorganic substances to a wet oxidation and/or catalytic wet oxidation treatment at least 100°C and at least 0.5 MPa, the method comprising the steps of: (1) recycling a portion of the liquid phase obtained by gas-liquid separation after a wet oxidation and/or catalytic wet oxidation treatment so as to mix the liquid phase with not-yet-treated alkaline wastewater in an amount according to the CODcr concentration of the not-yet-treated alkaline wastewater; and/or (2) mixing exhaust gas obtained from a gas-liquid separation after the wet oxidation and/or catalytic oxidation treatment with the not-yet-treated alkaline wastewater to precipitate salts, followed by separation of the salts before subjecting the alkaline wastewater to a wet oxidation and/or catalytic wet oxidation treatment.
Full Text DESCRIPTION
WASTEWATER TREATMENT METHOD
TECHNICAL FIELD
The present invention relates to a method of treatment of alkaline wastewater (hereinafter sometimes simply referred to as "wastewater") containing at least one nitrogen compound, organic substance or inorganic substance (hereinafter sometimes simply referred to as "pollutants").
BACKGROUND ART
Methods of wet oxidation and/or catalytic wet oxidation treatment of wastewater containing at least one nitrogen compound, organic substance or inorganic substance are known. For example, Japanese Examined Patent Publication No. 1984-29317 issued to the present applicant discloses "a method of decomposing ammonia, organic substances and inorganic substances in wastewater by subjecting the wastewater to a catalytic wet oxidation in the presence of a supported catalyst". This method can generally provide excellent wastewater treatment effects, as is clear from the results shown in the Examples therein.
However, when the wastewater has a high concentration of pollutant substances (e.g., a CODcr concentration of 75g/L or more and a TOD concentration of 83 g/L or more), this method requires treatment at high temperature-high pressure conditions using a relatively large amount of air (oxygen), so that a large amount of water in a reaction tower is vaporized and changes to the gas phase. To offset a temperature drop due to the latent heat of vaporization, external heating is necessary during the treatment operation. It may also become difficult to sufficiently maintain the wastewater in a liquid state to continue the reaction, thus decreasing the removal rate of pollutant components.
Furthermore, when a large amount of liquid evaporates from wastewater with a high concentration of pollutants, metals, metal salts (metal oxides, etc.), carbonates (Na2CO3, etc.), sulfates (Na2SO4, etc.) and the like in the wastewater become concentrated and metals in the wastewater may adhere to the surface of the carrier and/or catalyst and reduce their activity, thus resulting in insufficient treatment of the wastewater.
Additionally, when the alkaline wastewater containing pollutants has a high TOC concentration and/or a high total sulfur compounds concentration, precipitates such as alkaline salts and sulfur salts may cause clogging in the wet oxidation and/or catalytic wet oxidation treatment system, resulting in poor operation.
DISCLOSURE OF THE INVENTION Thus the object of the invention is to provide an efficient and reliable wet oxidation and/or catalytic wet oxidation treatment method for alkaline wastewater containing a high concentration of at least one species selected from the group consisting of nitrogen compounds, organic substances and inorganic substances (hereinafter sometimes simply referred to as "oxidation treatment").
More specifically, the object of the invention is to provide a novel oxidation treatment method which inhibits liquid evaporation in the treatment system and does not require external heating during operation, the method being capable of sufficiently maintaining the wastewater in a liquid state to continue the reaction, enabling reliable operation free from clogging by various salts in the system, efficiently treating even wastewater containing high concentrations of pollutants, and suppressing CO2 release into the atmosphere.
In view of the state of the prior art, the present inventors conducted extensive research and found that the above object can be achieved by recycling a portion of the liquid phase
after oxidation treatment and mixing it with not-yet-treated alkaline wastewater and subjecting the mixture to oxidation treatment, and by mixing the exhaust gas after oxidation treatment with not-yet-treated alkaline wastewater before oxidation treatment to precipitate salts, followed by removing the salts and then subjecting the resulting wastewater to an oxidation treatment. The inventors have conducted further research based on this finding and accomplished the invention.
Thus the present invention provides the following wastewater treatment methods:
Item 1. A method for treating alkaline wastewater comprising subjecting alkaline wastewater containing a high concentration of at least one species selected from the group consisting of nitrogen compounds, organic substances and inorganic substances to a wet oxidation and/or catalytic wet oxidation treatment at at least 100'C and at least 0.5 MPa, the method comprising the steps of:
(1) recycling a portion of the liquid phase obtained by gas-liquid separation after a wet oxidation and/or catalytic wet oxidation treatment so as to mix the liquid phase with not-yet-treated alkaline wastewater in an amount according to the CODcr concentration of the not-yet-treated alkaline wastewater; and/or
(2) mixing exhaust gas obtained from a gas-liquid separation after the wet oxidation and/or catalytic oxidation treatment with the not-yet-treated alkaline wastewater to precipitate salts, followed by separation of the salts before subjecting the alkaline wastewater to a wet oxidation and/or catalytic wet oxidation treatment.
Item 2. A method for treating alkaline wastewater according to item 1 wherein in step (1), when the amount of not-yet treated alkaline wastewater is W0 (kg/hr), the amount of the portion of the liquid phase recycled after gas-liquid separation is Wi (kg/hr), the CODcr concentration of the not-yet-treated alkaline
wastewater is X (g/L), and the relative amount of W1 to W0 is Y (i.e., Y = W1/W0) , then Y has a value within the range bounded by equations 1 and 2 in the correlation diagram between X and Y:
(Equation Removed) ………1 & 2
A method for treating alkaline wastewater according to item 1 wherein when the not-yet-treated alkaline wastewater has a TOC concentration of at least 15 g/L or a total sulfur compounds concentration of at least 30 g/L, and the entire amount of the exhaust gas obtained from the gas-liquid separation after the wet oxidation and/or catalytic oxidation treatment is mixed with the not-yet-treated alkaline wastewater.
The present invention will be described below in detail. The wastewater treated in the invention is not particularly limited as long as it is alkaline wastewater containing a high concentration of pollutants such as nitrogen compounds, organic substances and/or inorganic substances. The term "high concentration", as used herein means a CODcr concentration of at least 20 g/L, and particularly at least 30 g/L. The term "alkaline" as used in alkaline wastewater means a pH of about 8 to about 14. Wastewater with a pH outside the above range may suitably be adjusted to the above pH range by a known method. For example, wastewater can be prepared by adjusting to within the above pH range using sodium hydroxide, potassium hydroxide, or sodium carbonate.
Examples of nitrogen compounds in the wastewater include compounds containing NH4-N (ammonium nitrogen), NO2-N (nitrite nitrogen), NO3-N (nitrate nitrogen), organic nitrogen (e.g., amines), inorganic nitrogen (e.g., cyanides and thiocyanides), and the like.
Examples of organic substances in the wastewater include common organic substances (e.g., phenols, alcohols, aldehydes, carboxylic acids), chlorine-containing organic compounds (e.g., trichloroethylene, tetrachloroethylene, dioxins), and suspended matter (e.g., organic solid wastes, sludges
generated from various biological treatment processes, kitchen waste, municipal wastes, biomass-derived wastes).
Examples of inorganic substances in the wastewater

include common inorganic substances (e.g., S2O32- , S032- , SCN , CN ) .
Examples of wastewaters to be treated in the invention include wastewaters containing one species of the above nitrogen compounds, organic substances and inorganic substances, and wastewaters containing two or more such species thereof.
Specific examples of such wastewaters include wastewaters (gas liquors) generated from coke furnace plants, coal gasification plants, coal liquefaction plants and the like; wastewaters generated during gas generation in such plants; wastewaters from wet desulfurization towers and wet decyanation towers; photographic effluents; printing effluents; agricultural chemical wastewaters; dyeing effluents; semiconductor manufacturing factory wastewaters; organic synthetic chemical factory wastewaters; petrochemical plant wastewaters; petroleum refinery plant wastewaters; pharmaceutical factory wastewaters, paper manufacturing plant wastewaters; chemical plant wastewaters; domestic effluents containing kitchen garbage, paper and plastics; human wastes; wastewaters generated during thermal decomposition of municipal wastes; wastewaters generated during biological treatment of industrial wastewaters (anaerobic processing, aerobic processing), sewage sludges; wastewaters generated during oil conversion from sewage sludges; chlorinated organic compound-containing wastewaters; various types of cyanogen-containing wastewaters discharged from plating industries, cyanogen solutions used in steal soft nitriding, liquid carburizing, chemical conversion (or modification) and like surface treatments; cyanogen wastes discharged during such surface treatment processes; and the like.
The present invention is also useful for treating wastewaters and sludges containing, in addition to the above nitrogen compounds, organic substances (e.g. TOC) and inorganic substances, one or more metals such as Mg, Al, Si, P, Ca, Ti, Cr,
Mn, Fe, Co, N1, Cu, Zn and Cd.
The present invention provides a method of subjecting the above alkaline wastewater to a wet oxidation and/or catalytic wet oxidation treatment at at least 100°C and at least 0.5 MPa, the method comprising the following steps:
(1) recycling a portion of the liquid phase obtained by gas-liquid separation after a wet oxidation and/or catalytic wet oxidation treatment so as to mix the liquid phase with not-yet-treated alkaline wastewater in an amount according to the CODcr concentration of the not-yet-treated alkaline wastewater; and/or
(2) mixing exhaust gas obtained from a gas-liquid separation after the wet oxidation and/or catalytic oxidation treatment with the not-yet-treated alkaline wastewater to precipitate salts, followed by separation of the salts before subjecting the alkaline wastewater to a wet oxidation and/or catalytic wet oxidation treatment.
In step (1), a portion of the liquid phase after gas-liquid separation is recycled and mixed with the raw wastewater to be introduced into the reaction system, for example, at the inlet of the heater and/or inlet of the oxidation reactor.
The method for drawing a portion of the liquid phase after oxidation treatment may use the liquid phase from the lower portion of a high-temperature, high-pressure gas-liquid separator disposed at the outlet of the oxidation tower reactor, or may pass the oxidation-treated wastewater through a cooler and then through a gas-liquid separator to use the liquid phase from the lower portion of the separator.
According to step (1), since a portion of the liquid phase after gas-liquid separation is recycled and mixed with not-yet-treated alkaline wastewater, liquid evaporation in the treatment system can be suppressed, thus maintaining the liquid linear velocity in the oxidation reactor. Moreover, this step has the advantage that external heating is unnecessary during the
treatment operation and that a liquid state is well maintained to continue the oxidation treatment. Furthermore, since liquid evaporation is suppressed, the concentrations of metals and salts thereof in the wastewater do not increase, thus reducing metal adhesion and adsorption to the surface of the catalyst and prolonging catalytic activity.
When the amount of not-yet-treated alkaline wastewater introduced into the reaction system is W0 (kg/hr), the amount of the portion of the liquid phase recycled after gas liquid separation is Wi (kg/hr), the CODcr concentration of the not-yet-treated alkaline wastewater is X (g/L), and the relative amount of Wi to Wo is Y (i.e., Y = Wi/Wo) , the method for treating alkaline wastewater according to the invention is preferably such that in the correlation diagram between X and Y, Y falls within
the range bounded by equations 1 and 2 (see Fig. 4):

(Equation Removed)
By recycling and mixing a portion of the liquid phase after gas-liquid separation with not-yet-treated alkaline wastewater in an amount such that Y falls within the above-mentioned range, liquid evaporation can be suppressed, even when the oxidation treatment is performed under high-temperature, high-pressure conditions using a relatively large amount of air (oxygen), thus requiring no external heating and enabling the reaction to continue while sufficiently maintaining the wastewater in a liquid state.
In the correlation diagram between X and Y, when Y is smaller than the value shown by equation 1, pollutants in the not-yet-treated alkaline wastewater are insufficiently solubilized, resulting in deposition or precipitation thereof in the reaction system, thus being undesirable. On the other hand, when Y is larger than the value shown by equation 2, it is in the wastewater self-combusting range, so that recycling of such an excess of the liquid phase after gas-liquid separation does not

enhance the effects of the invention but simply increases costs, thus being undesirable.
In the correlation diagram between X and Y, it is particularly preferable that Y is within the range bounded by
equations 3 and 4:
(Equation Removed) 3 & 4
This is because when Y is within such a range, the above-mentioned effects of the invention are more suitably demonstrated.
The above relation is usually applied when the CODcr concentration of the not-yet-treated alkaline wastewater represented by X is at least 20 g/L. More specifically, when X is 30 g/L or more, preferably at least 40 g/L, more preferably about 50 to 500 g/L, and particularly about 70 to 350 g/L, the above relation is particularly suitably applied.
In step (2), exhaust gas obtained by gas liquid separation after the wet oxidation and/or catalytic oxidation treatment is introduced into not-yet-treated alkaline wastewater to precipitate salts. The salts thus precipitated are separated by sedimentation and the resulting wastewater (liquid phase) is then subjected to a wet oxidation and/or catalytic wet oxidation treatment. The method of introducing the exhaust gas into the raw wastewater is not particularly limited. The entire amount of exhaust gas may be recycled into the raw wastewater using a method such as a gas-liquid contact system, bubbling, or the like. The gas-liquid contact system may be a batch or continuous system.
The exhaust gas after oxidation treatment contains carbon dioxide generated by decomposition of organic substances. By introducing the exhaust gas into the not-yet-treated alkaline wastewater, the carbon dioxide-containing exhaust gas reacts with alkali metal ions, alkaline-earth metal ions and the like to form salts such as insoluble carbonates, sulfates and metal salts, and generates precipitates. By removing alkaline components and metals from the raw wastewater in such a proactive manner,
efficient treatment of wastewater can be achieved while suppressing any reduction of catalytic activity and preventing salts from precipitating in the reaction system, thus enabling reliable oxidation treatment.
In particular, when the not-yet-treated alkaline wastewater has a TOC concentration of at least 15 g/L (particularly 20 g/L or more) or a total sulfur compounds concentration of at least 30 g/L (particularly 35 g/L or more), the above remarkable effects are achieved.
Both the wet oxidation or catalyst wet oxidation according to the invention can be carried out using reaction conditions otherwise known except for the above steps (1) and (2).
For example, a wet oxidation tower reactor for use herein may be empty or have multi-level trays or have a carrier packed therein. Examples of usable carriers include at least one member selected from the group consisting of alumina, silica, zirconia, titania, and composite metal oxides containing such metal oxides (e.g., alumina-silica, alumina-silica-zirconia, titania-zirconia).
The catalytic oxidation tower reactor has a catalyst packed therein. Examples of active components of such a catalyst are at least one member selected from the group consisting of iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, copper, gold, tungsten, and water-insoluble or poorly water-soluble compounds of such metals. Composite catalysts containing a metal such as La, Ce and/or Te as an additional catalytic active component are also usable.
Examples of usable catalyst carriers are those mentioned in the above wet oxidation tower, i.e., at least one member selected from the group consisting of alumina, silica, zirconia, titania, and composite metal oxides containing such metal oxides (e.g., alumina-silica, alumina-silica-zirconia, titania-zirconia).
Composite catalyst carriers containing an additional
metal such as La, Ce and/or Te as an additional metal are also usable.
The carrier-supported catalyst used in the catalytic wet oxidation is not particularly limited and may be of various shapes such as spheres, pellets, cylinders, crushings, powders and honeycombs.
When such a carrier-supported catalyst is packed in a fixed-bed tower reactor, it is preferable that the reactor used
has a capacity such as to achieve a liquid space velocity of
-l -l
about 0.5 to 10 hr , and more preferably about 1 to 5 hr .
The carrier-supported catalyst used in such a fixed bed reactor usually has a particle size of about 3 to 50 mm, and preferably about 4 to 25 mm, when it is in the form of spheres, pellets, cylinders, crushings, powders or the like.
The carrier or catalyst (carrier-supported catalyst) preferably has the following physical property values: packing density: 1.2 g/cm ) or more; specific surface area: 10 m /g or more; pore volume: 0.10 cm /g) or more; compressive strength: 100 N or more.
With respect to a honeycomb carrier structure for supporting a catalyst, the opening of the honeycomb structures may be of any shape such as tetragonal, hexagonal, circular, etc.
The area per unit volume, aperture ratio, etc. are not particularly limited. A honeycomb structural body having an area
2 3
per unit volume of about 200 to 800 m /m and an aperture ratio of about 40 to 80% may usually be used.
Examples of materials for the honeycomb are metal oxides and metals such as those exemplified above. Highly durable zircoma, titania and titania-zirconia are particularly preferable.
The proportion of supported catalytic active components relative to the carrier is usually about 0.05 to 25 wt.%, and preferably about 0.3 to 3 wt.%.
The wet oxidation and/or catalytic wet oxidation treatment is usually carried out in a tower reactor at a temperature of at least 100°C and a pressure of at least 0.5 MPa, preferably at least 1 MPa. In particular, when the temperature is about 150 to 350°C, preferably about 200 to 300°C, and when the pressure is about 1 to 20 MPa, preferably about 2 to 15 MPa, the effects of the invention are particularly effective.
The higher the reaction temperature and pressure and the oxygen ratio of oxygen-containing gas are, the higher the rate of removal of pollutants by decomposition is, thus shortening the wastewater retention time in the reactor and reducing the amount of catalyst required but also increasing equipment and power costs. Therefore, the reaction temperature and pressure are set, within the above-mentioned range, in consideration of the pollutant concentration in the wastewater, the required level of treatment, operating costs, construction costs, etc.
The reaction pressure is usually 0.5 MPa or higher and is at least at a level that allows the liquid phase of the target wastewater to be maintained at the reaction temperature.
The phrase "pressure allows the liquid phase to be maintained", as used herein refers to a pressure at which the reaction tower contains not more than 60% water vapor (preferably 50% or less) as determined by the equilibrium calculation of the liquid (wastewater) amount, water vapor amount, and gas amount (amount of gases, other than water vapor, in the tower) at a specific reaction temperature and feed amount of oxygen-containing gas, thus substantially maintaining a liquid phase therein.
Oxygen is supplied to the tower reactor in at least the amount theoretically required to decompose the nitrogen compounds, organic substances and inorganic substances to harmless products. The amount of oxygen supplied is preferably 1 to 3 times, and particularly preferably about 1.05 to 1.5 times the theoretical
amount of oxygen.
Examples of usable sources of oxygen are atmospheric air, oxygen-enriched air (oxygen-enriched air obtained by using selective oxygen permeable membranes, air-oxygen mixtures, oxygen-enriched air obtained by treating air with a PSA (Pressure Swing Adsorption) device), molecular oxygen, and substances (e.g., O3, H2O2, etc.) capable of generating oxygen under the wastewater treatment conditions.
It is also possible to use as an oxygen source an oxygen-containing exhaust gas containing as impurities one or more of hydrogen cyanide, hydrogen sulfide, ammonia, sulfur oxides, organosulfur compounds, nitrogen oxides, and hydrocarbons. The impurities in such oxygen sources are also decomposed together with the wastewater pollutants to be treated by the method of the invention.
According to the method of the present invention, when a gas containing a high concentration of oxygen (e.g., pure oxygen) is used, wastewater treatment can be carried out under relatively low pressure conditions, e.g., at 10 kg/cm (0.98 MPa) or less and be completed in minutes.
When the wet oxidation and/or catalytic wet oxidation wastewater treatment is carried out using an oxygen-containing gas (e.g., H202) under subcritical, critical or supercritical conditions, the treatment can be completed in seconds.
The term "theoretical amount of oxygen" as used herein refers to the amount of oxygen necessary for decomposing the nitrogen compounds, organic substances and inorganic substances (target components) to harmless products (e.g., N2, H2O and CO2). The theoretical amount of oxygen can be easily determined by analyzing the target components in the wastewater and calculating the amount of oxygen required to decompose these components.
In practice, based on experience and experiments, one can find, by using various parameters, a relational expression that enables a highly accurate approximate calculation of the
theoretical amount of oxygen. Japanese Examined Patent Publication No. 27999/1983 describes an example of such a relational expression.
In a heat exchanger, heat is recovered by recycling a high-temperature gas-liquid mixture from a wet oxidation and/or catalytic wet oxidation tower reactor.
When the specified reaction temperature cannot be maintained due to heat dissipation in winter, etc. or when the temperature needs to be raised to a specific level, the temperature may be raised by a heat medium circulation (not shown) or external fuel heating (not shown), or by using steam from a steam generator (not shown).
It is also possible to directly supply high-pressure steam to the wet oxidation and/or catalytic wet oxidation tower reactor.
To adjust the reactor temperature to a specific level at the time of startup, the temperature can be raised by directly supplying steam to the tower reactor, or by using reaction heat generated by decomposing easily decomposable substances such as methanol after a certain temperature has been reached.
The present invention will be described below in more detail with reference to the accompanying drawings.
Fig. 1 is a flow diagram outlining the invention, illustrating one embodiment of the treatment process including both a wet oxidation tower reactor 9 and a catalytic wet oxidation tower reactor 109.
Wastewater is fed from a raw wastewater tank 1 by a pressure booster pump to increase the pressure to a certain level and then mixed with an oxygen-containing gas pressurized by a compressor 21. Subsequently, the wastewater is heated by a heat exchanger 5 and optionally a heater 7 to increase the temperature to a certain level and is then supplied to a wet oxidation tower reactor 9. The gas-liquid mixture from the wet oxidation tower
reactor 9 is then passed through the heat exchanger 5 and optionally through a cooler 30 and then separated into gas and liquid phases, i.e., exhaust gas and treated water, by a gas-liquid separator 16. The exhaust gas, containing carbon dioxide, is fed to the raw wastewater tank 1 so as to react with alkalis in the raw wastewater and precipitate carbonates, sulfates, metal salts, etc. The precipitates are removed as necessary from the lower portion of the tank 1. As mentioned above, the treated water is recycled in an amount according to the CODcr concentration of the not-yet-treated wastewater (wastewater after the wet oxidation treatment, i.e., wastewater in piping 102) and mixed with the raw wastewater.
Further, the treated water is fed from a wet oxidation treated water tank 101 by a pump 103 to increase the pressure to a certain level and then mixed with an oxygen-containing gas pressurized by a compressor 21. Subsequently, the water is heated by a heat exchanger 105 and optionally a heater 107 to increase the temperature to a certain level and then fed to a catalytic wet oxidation tower reactor 109. The gas-liquid mixture from the catalytic wet oxidation tower reactor 109 is then passed through the heat exchanger 105 and optionally a cooler 130 and then separated into gas and liquid phases, i.e., exhaust gas and treated water, by a gas-liquid separator 116. The exhaust gas, containing carbon dioxide, is fed to the raw wastewater tank 1 so as to react with alkalis in the raw wastewater and precipitate carbonates, sulfates, metal salts, etc. The precipitates are removed as necessary from the lower portion of the tank 1. As mentioned above, the treated water is recycled in an amount according to the CODcr concentration of the not-yet-treated wastewater and mixed with the raw wastewater.
The gas-liquid separation after the wet oxidation and/or catalytic wet oxidation treatment can be carried out not only in the manner mentioned above but also at the outlets of the wet oxidation tower reactor 9 and catalytic wet oxidation tower
reactor 109 under high-temperature, high-pressure conditions. The water thus separated may also be recycled.
At least a portion of the treated water obtained in each of the gas-liquid separator 16 and 116 in Fig. 1 is passed through a liquid recycling line 20 or 120 and a recycling pump 50 or 150 to return to the line upstream of pressure booster pump 3 or 103, respectively.
The amount of liquid phase recycled after gas-liquid separation Wi (Kg/hr) is determined according to the properties of the raw wastewater (e.g., the kind and concentration of components to be treated), the degree of activity reduction of the catalyst packed in the reactor, etc. The amount recycled is usually about 0.1 to 10 times, and preferably about 1 to 6 times the amount of raw wastewater W0 (kg/hr) . When the relative amount of Wi to W0 is Y (i.e., Y = Wi/Wo), Y can be determined according to the CODcr concentration of the alkaline wastewater. As mentioned above, in the correlation diagram between X and Y, it is preferable that Y is within the range bounded by equations 1 and 2 (and preferably equations 3 and 4).
When the amount of not-yet-treated alkaline wastewater introduced into the reaction system is W0 (kg/hr), the amount of the portion of the liquid phase recycled after gas liquid separation is W1 (kg/hr), the CODcr concentration of the alkaline wastewater is X (g/L), and the relative amount of W1 to W0 is Y (i.e., Y = W1/W0), the alkaline wastewater treatment method of the invention is characterized in that Y has a value within the range bounded by equations 1 and 2 (see Fig. 4).
To wash the catalyst while keeping a fixed bed in the reactor, the liquid linear velocity m the tower is usually about 0.1 to 1.0 cm/s., and preferably about 0.2 to 0.9 cm/s.
The gas linear velocity can be automatically determined based on the theoretical amount of oxygen according to the CODcr concentration (or TOD concentration) of the raw wastewater, in view of maintaining the above liquid linear velocity in the tower.
The mixing ratio of the exhaust gas after the wet oxidation and/or catalytic wet oxidation treatment to the not-yet-treated wastewater is determined in relation to the TOC concentration, alkali concentration and sulfate concentration of the wastewater. All or only a portion of exhaust gas may be recycled, and the gas-liquid contact may be by a batch or continuous method.
The inside of the reaction system may occasionally be washed with an acid or alkaline solution. Wash solution remaining in the reaction system during wet oxidation and/or wet catalytic oxidation treatment and any solution used for revitalizing the catalyst packed in the catalytic wet oxidation tower reactor may also be subjected, together with the wastewater, to the wet oxidation and/or catalytic wet oxidation treatment according to the method of the invention after removing metals, as necessary, from the solution by coagulation precipitation and/or the like.
The revitalization method for the catalyst is not particularly limited. The catalyst can be revitalized by a washing process alternately using gas-liquid mixtures, such as an aqueous acid solution-air mixture and/or aqueous alkaline solution-air mixture and/or aqueous acid solution-aqueous alkaline solution-air mixture.
Examples of aqueous acid solutions include aqueous nitric acid solution, aqueous ascorbic acid solution and the like. Examples of aqueous alkaline solutions include aqueous sodium hydroxide solution. Water washing is usually carried out between washing with an aqueous acid solution and washing with an aqueous alkaline solution.
Fig. 2 is a flow diagram illustrating one embodiment of the treatment process including a wet oxidation tower reactor 9.
The wastewater treated in the wet oxidation tower reactor 9 is separated into gas and liquid phases by a first high-pressure, high-temperature gas-liquid separator 11. A portion of the liquid phase separated thereby is passed through a
channel 12 and recycled (this recycling operation is referred to as "hot recycling") . As mentioned above, the rest of the gas-liquid mixture is used as a wastewater heating source in a heat exchanger 5 and, if necessary, passed through a cooler (not shown), and further fed to a second gas-liquid separator 16 to separate into a gas phase (exhaust gas) and a liquid phase (treated water). The exhaust gas, containing carbon dioxide, is passed through a channel 18 into the raw wastewater tank 1 to react with alkalis in the raw wastewater and precipitate carbonates, sulfates, metal salts, etc. The precipitates are removed as necessary from the lower portion of the raw wastewater tank 1.
Fig. 3 is a flow diagram illustrating one embodiment of the treatment process including a catalytic wet oxidation tower reactor 109.
Wastewater treated in a catalytic wet oxidation tower reactor 109 is separated into gas and liquid phases by a first high-pressure, high-temperature gas-liquid separator 111. A portion of the liquid phase separated thereby is passed through a channel 112 and hot recycled. As mentioned above, the rest of the gas-liquid mixture is used as a wastewater heating source in a heat exchanger 105 and, if necessary, passed through a cooler (not shown), and further fed to a second gas-liquid separator 116 to separate into a gas phase (exhaust gas) and a liquid phase (treated water). The exhaust gas, containing carbon dioxide, is passed through a channel 118 into the raw wastewater tank 101 to react with alkalis in the raw wastewater and precipitate carbonates, sulfates, metal salts, etc. The precipitates are removed as necessary from the lower portion of the raw wastewater tank 101.
EFFECT OF THE INVENTION The wet oxidation and/or catalytic wet oxidation treatment of alkaline wastewater containing a high concentration
of at least one species selected from the group consisting of nitrogen compounds, organic substances or inorganic substances (pollutants) according to the present invention is characterized by recycling a portion of the liquid phase obtained by gas-liquid separation after the wet oxidation and/or catalytic wet oxidation treatment. Based on this feature, the method can suppress liquid evaporation and maintain liquid linear velocity in the tower reactor, so that even when the treatment is performed under high-temperature high-pressure conditions using a relatively large amount of air (oxygen), external heating is unnecessary and the wastewater is sufficiently maintained in a liquid state to continue the reaction.
Furthermore, the method according to the invention prevents the concentrations of metals, carbonates (e.g., Na2CC>3) , sulfates (e.g., Na2S04) and the like in the wastewater from increasing by suppressing liquid evaporation, thus reducing adhesion or adsorption of metals to the surface of the catalyst and also reducing liquid film resistance at the surface of the catalyst, so that wastewater can be efficiently treated, irrespective of pollutant concentrations, while enhancing catalytic activity and durability.
Moreover, according to the method of the present invention, exhaust gas after wet oxidation and/or catalytic wet oxidation treatment is mixed with not-yet-treated wastewater to precipitate carbonates (e.g., Na2CO3) , sulfates (e.g., Na2SO4) and the like in the wastewater, followed by removing the salts therefrom and then subjecting the resulting wastewater to a wet oxidation and/or catalytic wet oxidation treatment. Therefore, the method prevents such salts from precipitating during the treatment operation, thus ensuring reliable treatment.
According to the method of the present invention, when using a gas containing a high concentration of oxygen (e.g., pure oxygen), wastewater treatment can be carried out under relatively low pressure conditions, e.g., at 10 kg/cm (0.98 MPa) or less and
be completed in minutes.
When the wet oxidation treatment and/or catalytic wet oxidation wastewater treatment is carried out using an oxygen-containing gas (e.g., H2O2) under subcritical, critical or supercritical conditions, the treatment can be completed in seconds.
Furthermore, according to the method of the invention, the steps of the process can be carried out continuously and result in an extremely simple treatment process, thus remarkably reducing treatment costs (e.g., equipment and running costs) and making process control easy.
BEST MODE FOR CARRYING OUT THE INVENTION Examples and Comparative Examples are provided below to describe the features of the invention in more detail.
Example 1
Wastewater with the properties shown in Table 1 discharged from a petroleum refinery plant (alkaline wastewater containing a high concentration of nitrogen-containing compounds, organic substances and inorganic substances) was treated according to the flow diagram of Fig. 1.
(Table Removed)
The CODcr concentration of the raw wastewater was 275 g/L, the amount of treated raw wastewater was 105 kg/hr, and the
amount of air was 125 Nm /hr. The wet oxidation treated water was recycled from the treated water tank 101 in an amount of 347 kg/hr for mixing with the raw wastewater. The amount recycled was about 3.3 times the amount of treated raw wastewater.
The outlet temperature of the wet oxidation reactor 9 was 275°C, and the pressure was 9.75 MPa. The steam ratio in the upper portion of the wet oxidation reactor 9 was about 50%. The reaction time was 1 hr.
The wet oxidation treated water (CODcr concentration: 65.9 g/L) thus obtained was subjected to a catalytic wet oxidation treatment to treat wastewater in an amount of 108 kg/hr using air in an amount of 35 Nm /hr. In the catalytic wet oxidation treatment, the catalytic wet treated water was not recycled from the treated water tank 140.
The outlet temperature of the catalytic wet oxidation reactor 109 was 270°C and the pressure was 9.75 MPa. The steam ratio in the upper portion of the catalytic wet oxidation reactor 109 was about 43%. A spherical, titania-supported ruthenium catalyst (diameter: about 5 mm) comprising 2 wt.% (based on the weight of the carrier) of ruthenium supported by a titania carrier was packed in the tower reactor 109 and used for treatment (reaction time: 2 hrs.).
As a result of these treatments, the catalytic wet oxidation treated water had a CODcr concentration of not more than 1 g/L. The CODcr and TOC removal achieved by the entire process including the above wet oxidation treatment and catalytic wet oxidation treatment was at least 99%. NH3-N was not detected in the treated water.
All of the exhaust gas after the wet oxidation treatment and the catalytic wet oxidation treatment was recycled to the raw wastewater and brought into gas-liquid countercurrent contact therewith, followed by removing alkaline salts (e.g., Na2C03), sulfur salts (e.g., Na2S04) and like precipitates and then subjecting the resulting wastewater to the oxidation
treatments, thereby ensuring long-term reliable operation. No salts appeared to be deposited or precipitated in the treatment reaction system.
Comparative Example 1
Treatment was carried out in the same manner as in Example 1 except that the exhaust gas after the wet oxidation treatment and catalytic wet oxidation treatment was not recycled. As a result, alkaline salts (Na2C03, etc.), sulfur salts (Na2S04, etc.) and the like were deposited on the piping, heat exchanger and tower reactor, thereby causing increased pressure loss and clogging and resulting in shutdown after only short-term use.
Comparative Example 2
Treatment was carried out in the same manner as in Example 1 except that no recycling from the wet oxidation treated water tank was performed in the wet oxidation treatment. As a result, due to a temperature drop in the wet oxidation reactor caused by liquid evaporation, and due to precipitation of metal salts, alkaline salts (Na2CO3, etc.) and sulfur salts (Na2S04, etc.), the CODcr concentration of the wet oxidation treated water was greatly increased, resulting in shutdown after a short period of time.
Example 2
Petroleum refinery plant wastewater (alkaline wastewater containing a high concentration of nitrogen-containing compounds, organic substances and inorganic substances) as used in Example 1 was diluted to a CODcr concentration of 100 g/1 and a TOC concentration of 36 g/L, followed by addition of ammonia water to achieve an NH3-N concentration of 3000 mg/L. This wastewater was treated according to the flow diagram of Fig. 3.
The amount of raw wastewater was 20.8 kg/hr, and the amount of air was 7.7 Nm/hr. The catalytic wet oxidation treated
water was recycled from the treated water tank in an amount of 31 kg/hr for mixing with the raw wastewater. The amount recycled was about 1.5 times the amount of raw wastewater. The reaction time was 0.75 hr.
The water quality after treatment was as follows: CODCr: 100 mg/L; TOC: 35 mg/L; and NH3-N: A spherical, titania-supported ruthenium catalyst (diameter: about 5 mm) comprising 2 wt.% (based on the weight of the carrier) of ruthenium supported by a titania carrier was packed in the tower reactor and used for treatment.
All of the exhaust gas after the catalytic wet oxidation treatment was recycled to the raw wastewater and brought into gas-liquid countercurrent contact therewith, followed by removing alkaline salts (e.g., Na2CO3) , sulfur salts (e.g., Na2SO4) and like precipitates and then subjecting the resulting wastewater to the oxidation treatment, thereby ensuring long-term reliable operation.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow diagram outlining a wet oxidation treatment and a catalytic wet oxidation treatment of the invention.
Fig. 2 is a flow diagram outlining a wet oxidation treatment of the invention.
Fig. 3 is a flow diagram outlining a catalytic wet oxidation treatment according of the invention.
Fig. 4 is a graph showing equations 1 to 4 in a correlation diagram between X and Y.
DESCRIPTION OF THE SYMBOLS 1: Raw wastewater tank 3: Pressure booster pump 5: Heat exchanger 7: Heater
9: Wet oxidation reactor
11: High-pressure high-temperature gas-liquid separator
16: Gas-liquid separator
17, 18: Wet oxidation exhaust gas
19, 20: Wet oxidation treated water
30: Cooler
50: Recycling pump
101: Wet oxidation treated water tank
103: Pressure booster pump
105: Heat exchanger
107: Heater
109: Catalytic wet oxidation reactor
111: High-pressure high-temperature gas-liquid separator
116: Gas-liquid separator
117, 118: Catalytic wet oxidation exhaust gas
119, 120: Catalytic wet oxidation treated water
121: Compressor
139: Cooler
140: Treated water tank
150: Recycling pump






WE CLAIM
1. A method for treating alkaline wastewater comprising subjecting alkaline wastewater
containing a high concentration of at least one species selected from the group
consisting of nitrogen compounds, organic substances and inorganic substances to a
wet oxidation and/or catalytic wet oxidation treatment at least 100°C and at least 0.5
MPa,
the method comprising the steps of:
(1) recycling a portion of the liquid phase obtained by gas-liquid separation after a
wet oxidation and/or catalytic wet oxidation treatment so as to mix the liquid phase
with not-yet-treated alkaline wastewater in an amount according to the CODcr
concentration of the not-yet-treated alkaline wastewater; and/or
(2) mixing exhaust gas obtained from a gas-liquid separation after the wet oxidation
and/or catalytic oxidation treatment with the not-yet-treated alkaline wastewater to
precipitate salts, followed by separation of the salts before subjecting the alkaline
wastewater to a wet oxidation and/or catalytic wet oxidation treatment.
2. A method for treating alkaline wastewater as claimed in claim 1 wherein in step (1),
when the amount of not-yet-treated alkaline wastewater is Wo (kg/hr), the amount of
the portion of the liquid phase recycled after gas-liquid separation is W1 (kg/hr), the
CODcr concentration of the not-yet-treated alkaline wastewater is X (g/L), and the
relative amount of W1 to Wo is Y (i.e., Y = W1/W0), then Y has a value within the
range
bounded by equations 1 and 2 in the correlation diagram between X and Y:
(Equation Removed)
3. A method for treating alkaline wastewater as claimed in claim 1 wherein in step (2) when the not-yet-treated alkaline wastewater has a TOC concentration of at least 15 g/L or a total sulfur compounds concentration of at least 30 g/L, and the entire amount of the exhaust gas obtained from the gas-liquid separation after the wet oxidation and/or catalytic oxidation treatment is mixed with/the not-yet-treated alkaline wastewater.

Documents:

698-DEL-2005-Abstract-(23-02-2011).pdf

698-del-2005-abstract.pdf

698-DEL-2005-Claims-(23-02-2011).pdf

698-del-2005-claims.pdf

698-DEL-2005-Correspondence-Others-(23-02-2011).pdf

698-del-2005-Correspondence-Others-(28-04-2011).pdf

698-del-2005-correspondence-others.pdf

698-del-2005-description (complete).pdf

698-del-2005-drawings.pdf

698-del-2005-form-1.pdf

698-del-2005-form-18.pdf

698-del-2005-form-2.pdf

698-DEL-2005-Form-3-(23-02-2011).pdf

698-del-2005-Form-3-(28-04-2011).pdf

698-del-2005-form-3.pdf

698-del-2005-form-5.pdf

698-del-2005-gpa.pdf

698-DEL-2005-Petition 137-(23-02-2011).pdf

698-del-2005-petition-138.pdf


Patent Number 247427
Indian Patent Application Number 698/DEL/2005
PG Journal Number 14/2011
Publication Date 08-Apr-2011
Grant Date 07-Apr-2011
Date of Filing 30-Mar-2005
Name of Patentee OSAKA GAS CO., LTD.
Applicant Address 1-2, HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU, JAPAN.
Inventors:
# Inventor's Name Inventor's Address
1 YOSHIAKI HARADA NO. 23 OF SOUTH ROAD OF XIN WEN, KUNMING CITY, YUNNAN PROVINCE, P.R. CHINA.
2 KENICHI YAMASAKI C/O OSAKA GAS CO., LTD., 1-2, HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU, JAPAN.
3 TAKAYUKI AZUMA C/O OSAKA GAS CO., LTD., 1-2, HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU, JAPAN.
4 BIAO QIAN NO.23 OF SOUTH ROAD OF XIN WEN, KUNMIN CITY, YUNNAN PROVINCE, P.R.CHINA.
5 YING YANG NO.23 OF SOUTH ROAD OF XIN WEN, KUNMIN CITY, YUNNAN PROVINCE, P.R.CHINA.
6 GUANG HUI ZHAO NO.23 OF SOUTH ROAD OF XIN WEN, KUNMIN CITY, YUNNAN PROVINCE, P.R.CHINA.
PCT International Classification Number C02F 1/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2004-107996 2004-03-31 Japan