Method for treating waste by hydrothermal oxidation

[0001] The present invention relates to waste present in wastewater, in particular, but not limited to, methods of thermally hydroxylating organic substances, and plants used to carry out the above methods. [0002] The use of the present invention is, but is not limited to, the conversion of organic substances present in a small amount in wastewater generated from the food processing industry in particular. These wastewaters may also contain dissolved salts. Organic matter is converted to a gas that can be incinerated to provide energy, or to a gas that can be released into the atmosphere without danger. [0003] Methods for converting organic waste present in the aqueous phase are known. Especially when oxidizing substances are present in the water / organic waste mixture, the mixture is brought to a temperature and pressure above the critical point of water, thereby causing the waste to CO.₂And H₂It is known to decompose into simple O-type chemical components. [0004] However, when a water / organic waste mixture containing an amount of oxidant capable of oxidizing all waste is heated and pressurized to exceed the critical point of water, the oxidative reaction generated results in a large amount of heat. Generate energy. This large amount of thermal energy can affect the integrity of the reactor wall where the reaction takes place. The same results are observed for the reactor when the oxidant is introduced into the mixture after pressurizing and heating the water / organic waste mixture. [0005] On the other hand, when the mixture is pressurized and heated after the introduction of the oxidant, hot spots may appear in the reactor. This is essentially due to the fact that the solubility of the oxidant and its heat capacity are not constant depending on the temperature and pressure conditions of the mixture. Therefore, the concentration of oxidant dissolved in the mixture is not homogeneous in the reaction medium, and the oxidative reaction produces more thermal energy in the region where the oxidant concentration is higher. [0006] In addition to the fact that the appearance of these hotspots can affect the reactor wall, the poor distribution of oxidants in the reaction medium reduces the yield of organic waste decomposition reactions. [0007] To overcome localized overheating of the reactor, oxygen and water are injected simultaneously and differentially along the reactor, whereby oxygen oxidizes organic matter and at the same time water lowers the temperature of the reaction medium. I am trying to do it. However, this solution does not optimize the decomposition of organic matter, as the rate of oxidation decreases with simultaneous temperature reduction. In addition, the thermal structure of the reactor shows a curve that alternates with each injection and then decreases, reducing the overall yield of the reactor. [0008] A first object of the present invention is to provide a method for oxidizing an organic compound in wastewater that overcomes the above-mentioned drawbacks. [0009] According to the present invention, this object is achieved by the following method. [0010] This method of the present invention Wastewater containing a predetermined amount of organic matter collected under initial pressure and temperature conditions is injected into a tubular body with inlets and outlets. The wastewater is set to a pressure P1 that is at least corresponding to the critical pressure of this wastewater and is higher than the initial pressure. The heating means applied to a particular region of the tubular body brings the wastewater to a temperature T1 above the initial temperature. Including things, as well as N fractions of at least one oxidative composition, the total amount of which corresponds to the amount required to oxidize the predetermined amount of organic material, are placed into a tubular body at n locations spaced apart from each other. A portion of the heat energy generated by the injection and the oxidation reaction reacts from the temperature T1 to a temperature T2 higher than this temperature T1 according to an increasing curve between the region of the tubular body and the nth injection site. It is characterized by raising the temperature of the mixture, thereby oxidizing the organic material and continuously changing the reaction mixture from the subcritical liquid state to the supercritical region. [0011] One feature of this method of oxidizing organic matter is that only the oxidizing composition is injected stepwise into the reaction medium flowing through the tubular body through n injection points. In this method, the oxidation of the organic matter present in the waste water is carried out stepwise as the reaction mixture flows through the tubular body, and the thermal energy generated by the oxidation reaction at each injection of the oxidizing composition is at the injection site. It is partially dispersed between them, thereby preventing excessively strong energy generation that damages the inner wall of the tubular body. Also, it is not necessary to inject a substance capable of simultaneously cooling the reaction medium during these reactions. [0012] As will be very apparent, the oxidizing composition can consist of other compounds having no particular effect on the reaction medium. [0013] Part of the total thermal energy produced by all organic substances is imparted to the reaction medium. Further, the pressure P1 of the reaction medium is higher than the critical pressure of the wastewater, so that the reaction medium gradually changes from the subcritical state to the supercritical region in the liquid phase without passing through the gas phase. When the reaction mixture is in the supercritical region, the concept of phase disappears and the unoxidized organic matter between the injection sites of the oxidizing composition is oxidized in this region. [0014] Fortunately, the pressure P1 of the wastewater is higher than 23MPa, and the temperature T1 of the wastewater is 370 ~ 52.<u style="single">0K</u>Is. In this temperature and pressure region, the wastewater containing an organic substance is in a subcritical liquid phase state, in which a part of the organic substance is oxidized. [0015] According to a particular aspect of the invention, some of the thermal energy produced by the oxidation reaction raises the temperature of the reaction mixture to 80.<u style="single">0K</u>Raise to a lower temperature T2. After the nth injection point, the temperature of this mixture is 80<u style="single">0K</u>However, this amount of energy is not enough to damage the inner wall of the tubular body, as the nth fraction of the oxidizing composition reacts with the remaining organic matter. The temperature of the reaction mixture is higher than T2 because the reaction mixture reaches temperature T2 after a substantial portion of the organic material has been oxidized and less final part is oxidized by the final fraction of the oxidizing composition. Is only very slightly higher. In addition, the heat capacity of water is 65<u style="single">0K</u>And 70<u style="single">0K</u>As the reaction medium passes through this temperature range, the thermal energy generated by the oxidation reaction is significantly absorbed. The effect on the reactor wall decreases proportionally. [0016] According to another particular aspect of the invention, the three fractions of the oxidizing composition are injected into the tubular body at three locations spaced apart from each other. After the wastewater reached temperature T1, the first fraction of the oxidizing composition was injected and the second fraction was injected to allow the wastewater to reach temperature T2 and the wastewater reached temperature T2. Occasionally inject a third fraction. [0017] Conveniently, after the wastewater reaches the temperature T1, the first fraction of the oxidizing composition is injected, thereby causing the oxidation reaction of the organic material only downstream of the area of the tubular body to which the heating means has been applied. Try to get started. [0018] According to another particular aspect of the invention, the tubular body has multiple portions with different cross-sectional dimensions. This shape allows the alternating insertion of narrower and wider portions of the tubular body, injecting the oxidizing composition in this narrow portion and allowing the oxidation reaction to take place in the wider portion. Thereby, the residence time of the reaction mixture in a wider portion can be increased, the reaction time can be increased, and the reaction yield between each injection site of the oxidizing composition can be increased. [0019] According to the favorable arrangement, a part of the thermal energy generated by oxidation is applied to the wastewater collected under the initial pressure and temperature conditions to bring the temperature of the wastewater to the temperature T1. According to this method, it is not necessary to provide a heating means for bringing the wastewater from the initial temperature to the temperature T1. This improves the total energy balance of the method according to the invention. In this case, only a low intensity starting heating means is required. [0020] Preferably, the oxidizing composition injected into the tubular is oxygen, which allows the organic material to be converted at a favorable cost. However, if the conditions under which this method is carried out require an oxidizing composition that is highly soluble in water, hydrogen peroxide can be used in certain circumstances where supply costs are favorable. [0021] [0021] According to a particularly favorable arrangement, at least one of the fractions of the oxidizing composition is composed of an oxidizing composition whose properties differ from the other fractions. Thereby, for example, in the first part of the reactor it is possible to benefit from the technical benefits of hydrogen peroxide and in the second part it is possible to benefit from the cost benefits of oxygen. [0022] According to a particularly convenient embodiment, the method according to the invention Collect the wastewater and the salts present in it at the outlet of the tubular body, The pressure of the wastewater is reduced from the pressure P1 to the pressure P0 between the atmospheric pressure and the pressure P1, thereby making all the salts solid and the wastewater steam; Recover the salts in solid form; The wastewater is recovered in a steam state, thereby physically separating the wastewater and the salts present therein. It also includes:. [0023] A second object of the present invention is to provide a plant for oxidizing organic substances existing in wastewater. [0024] This plant A means of injecting wastewater containing a predetermined amount of organic matter collected under initial pressure and temperature conditions into a tubular body having inlets and outlets. Means to make wastewater pressure P1 higher than initial pressure, A heating means, and a heating means, applied to a specific area of the tubular body to bring the wastewater to a temperature T1 above the initial temperature. At least the pressure in the tubular body in n fractions of the oxidizing composition, the total amount of which corresponds to the amount required to oxidize the predetermined amount of the organic material, at n locations spaced apart from each other. A part of the heat energy generated by the oxidation reaction by injecting into the waste water of P1 is from temperature T1 to a temperature higher than this temperature T1 according to an increasing curve between the region of the tubular body and the nth injection point. A means of raising the temperature of the reaction mixture to T2, thereby oxidizing the organic material and continuously moving the reaction mixture from the subcritical liquid state to the supercritical region. Contains. [0025] The tubular body is advantageously composed of a tube having an inlet orifice for injecting wastewater and an outlet orifice for extracting oxidized organic matter. If this method can be performed in a short tubular body, the tubes may be straight, but the tubes can be arranged in a spiral to reduce the overall size of the reactor. [0026] Preferably, the means for injecting the wastewater comprises a pump capable of pressurizing the wastewater to a pressure higher than 23 MPa, which pump is connected to the inlet orifice. This pump has an inlet orifice for wastewater and an orifice for pressurizing and injecting wastewater into a tubular body. The pressure of the wastewater in the tubular body is relatively constant, at least in the part where the oxidation reaction occurs, and is higher than 23 MPa. [0027] The heating means applied to a particular area of the tubular body preferably comprises an electric heater that is integral with the tubular body. Thereby, the injected wastewater can be preheated by an electric heater attached to the tubular body. [0028] The heating means applied to a particular region of the tubular body preferably comprises a heat exchanger that is integral with the tubular body and the heat source of this heat exchanger is provided by a portion of the thermal energy generated by the oxidation reaction. Will be done. This is because the oxidation reaction produces thermal energy, at least part of which can raise the temperature of the reaction medium, and at least part of it can be recovered and used to bring the wastewater to temperature T1. [0029] According to a particular aspect of the invention, the means for injecting the oxidant fraction into the tubular body comprises a variable flow rate injection device in the tubular body, the pressure of the oxidant in this injection device being higher than P1. Injecting the oxidizing composition into the injection device via a pump capable of pressurizing the oxidizing composition to a pressure higher than P1 or also via a tank holding the oxidizing composition under a pressure higher than P1. Can be supplied. [0030] According to a particular aspect of the invention, the means for injecting the oxidant into the tubular body comprises three injecting devices spaced apart from each other in the tubular body. [0031] The first location for injecting the oxidizing composition is preferably located between the outlet orifice of the tubular body and the area of the tubular body to which the heating means is applied, close to this area. [0032] According to a particular aspect, the oxidation plant A means of recovering wastewater and salts present therein at the outlet of a tubular body, A means of reducing the pressure of wastewater from pressure P1 to pressure P0 between atmospheric pressure and pressure P1, thereby solidifying all salts and steaming the wastewater. Means for recovering salts in a solid state, and A means of recovering wastewater in a steam state, thereby physically separating the wastewater from the salts present in it. Is further included. [0033] Other advantageous and unique features of the invention will become apparent when reading the description below for particular aspects of the invention with reference to the accompanying drawings. However, these are examples and are not limited. [0034] Figure 1 shows a plant that implements a method of oxidizing organic substances present in wastewater. [0035] Wastewater containing organic matter to be converted is stored in tank 10 upstream of the plant performing this method. Wastewater is generally composed of industrial or urban sludge, or an aqueous liquid resulting from an industrial method. [0036] The pump 12 can inhale and drain the wastewater and inject it under pressure into the tubular body 18 at the inlet orifice 20. Here, the inlet orifice 14 of the pump 12 is connected to the lower end of the tank 10 via a pipe 16. Pump 12 can inject wastewater into the tubular body 18 under pressures above 22 MPa. This pressure of 22 MPa corresponds substantially to the critical pressure of water. [0037] The tubular body 18 is equipped with an electric heater 22. The electric heater 22 surrounds the outer wall of the tubular body at least partially near the inlet orifice 20 into which the wastewater is injected. The electric heater 22 is composed of a resistance heater capable of generating sufficient thermal energy to raise the temperature of the wastewater passing through the tubular body 18. [0038] It goes without saying that any other means capable of generating thermal energy, in particular means that can be operated with gas or other fuels, can be used. [0039] This energy contribution to wastewater is necessary to initiate the oxidation reaction of organic matter. This oxidation reaction occurs as soon as the first fraction of the oxidizing composition is injected into injection site 24. The injection point 24 is located in a tubular body 18 downstream of the electric heater 22. In certain embodiments, the first fraction of the oxidizing composition is injected upstream of the heating means after the inlet orifice of the tubular body, whereby part of the oxidizing composition dissolves in the aqueous phase at initial temperature. To do. [0040] At the injection site 24, an injection device (not shown) appears in the tubular body 18 through the wall of the tubular body 18. The injection device is connected to the pump 26 or tank (not shown) by a pipe 28. The pump 26 or tank can deliver a fraction of the oxidizing composition at a pressure greater than the pressure of the moving effluent in the tubular body 18. This is because this condition is necessary to inject the oxidant into the tubular body 18. [0041] The oxidizing composition can be composed of any substance capable of taking electrons from an organic substance. The cheapest oxidant is oxygen, which is easy to inject with an infusion device. Other oxidants such as hydrogen peroxide or nitric acid can be used, which has the advantage of being able to decompose nitrogen oxides to produce water and nitrogen. [0042] The second site 30 for injecting the oxidizing composition is located near the first injection site and downstream thereof, allowing injection of a second fraction of the oxidizing composition. The means used to inject the oxidizing composition are the same as those used to carry out the injection at location 24. [0043] The number of fractions of the oxidizing composition to be injected into the tubular body 18 is a function of the concentration of organic matter present in the wastewater and the amount of oxidant required to oxidize all the organic matter, and of the tubular body. It can be changed as a function of shape. Specific embodiments of the invention in which the plant has three oxidizing composition injection sites will be described in more detail below. [0044] According to a convenient arrangement, if the temperature of the reaction medium is raised after the injection of the first fraction of the oxidant, at least two types of oxidants are used. Hydrogen peroxide, which has high oxidizing power, is injected first, and then a fraction of oxygen is injected at other injection points. Once the reaction begins, oxygen can react in an optimal manner. According to this aspect, the cost balance of the reactor is improved because oxygen is cheaper than hydrogen peroxide. [0045] According to FIG. 1, the plant has a final point 32 for injecting the oxidizing composition, known as the nth injection point. [0046] In order to substantially complete the oxidation reaction, that is, to oxidize all the organic substances, an amount of an oxidizing agent that is at least equal to the amount of the oxidizing agent corresponding to the chemical amount of the oxidation reaction of the organic substances is added. It is necessary to inject it into waste water. Thereby, the total fraction of the oxidizing composition injected into the tubular body 18 is at least equal to the stoichiometric amount of oxidant required for the oxidation reaction of a predetermined amount of wastewater. As is very clear, the oxidation process occurs continuously, and the results applied for a given amount can be applied to continuous operations by using flow rate measurements. [0047] When the reaction is complete and the organic matter contains only carbon and oxygen based compounds, the oxidation product is composed of carbon dioxide and water. These oxidation products are released from the ends of the tubular body 18 at the outlet orifice 34. [0048] The method according to the invention makes it possible to convert organic matter present in wastewater into inorganic compounds, such as producing water and carbon dioxide. In this case, the reaction product can be reliably released into the atmosphere without harming the environment, or if the carbon dioxide content is sufficient, it can be recovered for use as a reaction component. [0049] Products from the oxidation reactions of organic substances can also be released into the atmosphere, for example, if those products contain nitrogen resulting from the decomposition of nitrogen oxides by nitric acid. On the other hand, when the organic substance contains chlorine, the hydrogen chloride generated from the reaction must be recovered by chemical conversion. [0050] As will be described in more detail below, it is speculated that the tubular body is at maximum temperature in the region located after the nth injection site. Therefore, this thermal energy can be recovered by the first exchanger 36 located in this region where the temperature is highest and transferred by the second exchanger 38 upstream of the tubular body 18. This thermal energy is transmitted near the inlet orifice 20 of the tubular body 18 and can assist or replace the electric heater required to preheat the wastewater. This arrangement has economic advantages in reducing the amount of energy required to carry out this method. [0051] Since the components of the plant necessary for carrying out the method according to the present invention have been described with reference to FIG. 1, the method for oxidizing organic substances existing in the waste liquid and the thermal fluctuation of the reaction medium are then referred to with reference to FIG. Will be explained. This FIG. 2 is shown directly below the device of FIG. 1 so that the thermal variation of the reaction medium corresponds to the various parts of the tubular body 18. [0052] After first pressurizing the wastewater with pump 12, 22 Inject into the inlet orifice 20 of the tubular body 18 under a pressure greater than MPa. The pressurization that allows the temperature of the wastewater to rise is assisted by the second exchanger 38 when the plant is under normal operating conditions, or by the thermoelectric energy generator 22 when the plant is in a transient state. Will be done. The reaction medium is initially at temperature Ti, which is brought to temperature T1 along slope 40 according to the thermal change in FIG. [0053] While the pressure of the reaction medium is kept constant, the temperature T1 is 370-52.<u style="single">0K</u>This keeps the reaction medium in a liquid phase state. The reaction medium maintains a constant temperature T1 for a transient period corresponding to the flat region 42. [0054] Subsequently, the first fraction of the oxidizing composition is injected at the first injection point 24, and the temperature of the reaction medium rises with slope 44 to temperature T1.₁To reach. This is because the oxidation of an organic substance by an oxidizing composition is exothermic and ultimately imparts energy to the reaction medium. [0055] The second fraction of the oxidizing composition is injected at the second injection site 30 and the value T1 according to the slope 46.₂Produces energy that can raise the temperature. [0056] The same operation is repeated as many times as necessary to control and inject the fraction of the oxidizing composition to limit the temperature of the reaction medium. [0057] The temperature of the reaction medium must not be higher than the temperature T2 before injecting the nth fraction of the oxidant into the tubular body 18 at the injection site 32. Where this temperature T2 is 80<u style="single">0K</u>Lower. This is because otherwise, the nth and final injections will further increase the temperature of the reaction medium according to the slope 48, and there is a high risk of damage to the inner wall of the tubular body 18. [0058] [0058] The final injection of the oxidizing composition allows the decomposition of organic matter in the wastewater that was not decomposed during the preceding steps. To maximize the yield of the oxidative reaction, the sum of the n fractions of the oxidative composition is substantially greater than the stoichiometrically required amount. Very clearly, when this method is continuous, the sum of the flow rates of the oxidizing composition fractions with respect to the flow rate of wastewater in the tubular body 18 is greater than the stoichiometric amount. [0059] In addition, the heat capacity of water is 67<u style="single">0K</u>This 67 because it is maximal at temperatures substantially equal to<u style="single">0K</u>Within the temperature range of the reaction medium containing the value of, a larger amount of the oxidizing composition is conveniently injected. Because the heat capacity of water is maximum at this temperature, the thermal energy produced by the oxidation reaction is absorbed very well, which limits the temperature rise of the reaction medium, thereby damaging the inner wall of the tubular body 18. Because it limits. [0060] Furthermore, when the oxidizing composition is oxygen, it is soluble in the liquid phase of wastewater for all infusions. This favorable and unique feature allows hot spots in the tubular body to be avoided. This is because the complete dissolution of oxygen in the reaction medium allows for homogeneous and instantaneous distribution of the oxidant, thereby initiating the reaction at substantially the same time, raising the temperature of the reaction medium as a whole. is there. Conversely, if the oxidant is less soluble, the reaction will be localized in the reaction medium and thus hot spots will occur. [0061] In FIGS. 3 and 4, these drawings describe a particular embodiment having three injection points for three fractions of the oxidizing composition. [0062] The plants according to the invention and their associated thermal changes are shown in FIGS. 3 and 4, respectively. Wastewater is injected into the inlet orifice 20 under pressure. The injection of the first fraction of the oxidizing composition at the preheating means and the injection site 24 allows the reaction medium to reach the temperature T1 corresponding to the plateau 50 during the transient period. When the second fraction of the oxidizing composition is injected at the injection site 30, the temperature rises to the value T2 corresponding to the plateau 52. Subsequently, the final injection allows the oxidation of unreacted organic matter, raising the temperature of the reaction medium to a temperature substantially higher than T2. As is very clear, the values of T1 and T2 are in this case the same as the values T1 and T2 described in FIGS. 1 and 2. [0063] According to other particular embodiments, this is not shown, but the tubular body upstream of the preheating means near the inlet orifice of the tubular body, while retaining the aforementioned principle of injecting three fractions of oxidant. The first fraction is injected into the injection site located inside. Thereby, the oxidizing composition constitutes a reaction medium having a temperature substantially equal to the initial temperature of the wastewater together with the wastewater containing an organic substance. The preheating means can initiate the oxidation reaction from the initial temperature rise of the reaction medium, which temperature rise is also brought about by the reaction. [0064] According to a further aspect (not shown), only two fractions of the oxidizing composition are injected. This arrangement is advantageous when the concentration of organic matter in the wastewater is low. [0065] Specific examples of carrying out the present invention are illustrated below. [0066] The reactor or tubular body has four injection points and a temperature of wastewater of 42.<u style="single">5K</u>Includes a preheater that can reach the temperature of. [0067] The waste liquid to be treated is composed of a mixture of glucose and methanol containing 3.9% by weight glucose and 4.9% methanol in the aqueous phase. To completely oxidize this mixture, the oxygen requirement is 88.9 g / l. This amount is known as "Chemical Oxygen Demand" or commonly known as COD. The injection amount in this case corresponds to 1.1 times the stoichiometric amount. [0068] The flow rate of waste liquid in the reactor is 1 kg / hour at a pressure of 25 MPa. [0069] The measured reactor length (meters) (the reference position is essentially the point where the waste liquid is injected), the oxygen fraction injection position, and the corresponding temperature of the reactor are listed in the table below. [0070] [table 1]<img file="JP4852222B2_D0001.tif" />[0071] The above examples are not marginal at all, and treating any other waste composition by plants using different oxidizing agents and having different numbers of injection sites would not deviate from the scope of the invention. .. [0072] On the other side, the oxidation plant comprises means (not shown) for recovering the salts present in the wastewater. [0073] In this case, the tubular body is expanded by the second tubular body at its exit. In the second tubular body, wastewater and salts existing in it are 750 to 90<u style="single">0K</u>, For example 82<u style="single">0K</u>Inflow at the temperature of. The second tubular body has an inlet nozzle, in which water is injected into this nozzle to drain 700-80 wastewater.<u style="single">0K</u>, For example 75<u style="single">0K</u>Can be cooled to the temperature of. [0074] The second tubular body appears in the hopper forming vessel through the pressure drop nozzle. The internal pressure of this container is between atmospheric pressure and the pressure P1, for example, 1 MPa. In that method, the pressure of the wastewater containing salts is reduced, all the salts are made into a solid state, and the wastewater is made into a steam state. Thereby, salts can be recovered at the lower end of the hopper and vapor can be recovered at temperatures of 500-600 ° K, for example 550 ° K, at other outlets inserted for this purpose. [0075] Further, in a particularly convenient manner, the tubular body outlet and / or the second tubular body comprises an ultrasonic cleaning device applied to the outer wall. The ultrasonic cleaning device can settle on the inner wall of the tubular body and remove salts that can block the tubular body during the oxidation process. [Simple explanation of drawings] FIG. 1 is a diagram of a plant carrying out the method of the invention, having n injection points of the oxidizing composition. FIG. 2 is a diagram showing a thermal change in the reaction medium with respect to the injection site of the oxidizing composition corresponding to the diagram of FIG. [Fig. 3] FIG. 3 is a diagram of a plant in which the method according to the invention is carried out in a particular embodiment, where the tubular body has three injection points. FIG. 4 is a diagram showing thermal changes in the reaction medium with respect to the injection site corresponding to the diagram of the plant shown in FIG.